<title>The Virtual Table Mechanism Of SQLite</title>

<tcl>hd_keywords {virtual table} {virtual tables}</tcl>

<table_of_contents>

<h1>Introduction</h1>

<p>A virtual table is an object that is registered with an open SQLite
[database connection]. From the perspective of an SQL statement,
the virtual table object looks like any other table or view. 
But behind the scenes, queries and updates on a virtual table
invoke callback methods of the virtual table object instead of
reading and writing on the database file.

<p>The virtual table mechanism allows an application to publish
interfaces that are accessible from SQL statements as if they were
tables. SQL statements can do almost anything to a
virtual table that they can do to a real table, with the following
exceptions:

<p>
<ul>
<li> One cannot create a trigger on a virtual table.
<li> One cannot create additional indices on a virtual table. 
     (Virtual tables can have indices but that must be built into
     the virtual table implementation.  Indices cannot be added
     separately using [CREATE INDEX] statements.)
<li> One cannot run [ALTER TABLE | ALTER TABLE ... ADD COLUMN]
     commands against a virtual table.
</ul>

<p>Individual virtual table implementations might impose additional
constraints. For example, some virtual implementations might provide
read-only tables. Or some virtual table implementations might allow
[INSERT] or [DELETE] but not [UPDATE].  Or some virtual table implementations
might limit the kinds of UPDATEs that can be made.

<p>A virtual table might represent an in-memory data structures. 
Or it might represent a view of data on disk that is not in the
SQLite format. Or the application might compute the content of the 
virtual table on demand.

<p>Here are some existing and postulated uses for virtual tables:

<ul>
<li> A [full-text search] interface
<li> Spatial indices using [R-Trees]
<li> Introspect the disk content of an SQLite database file
     (the [dbstat virtual table])
<li> Read and/or write the content of a comma-separated value (CSV)
     file
<li> Access the filesystem of the host computer as if it were a database table
<li> Enabling SQL manipulation of data in statistics packages like R
</ul>


<h2>Usage</h2>

<p>A virtual table is created using a [CREATE VIRTUAL TABLE] statement.

<tcl>
RecursiveBubbleDiagram create-virtual-table-stmt
</tcl>

<p>The CREATE VIRTUAL TABLE statement creates a new table
called <yyterm>table-name</yyterm> derived from the class
class <yyterm>module-name</yyterm>.  The <yyterm>module-name</yyterm>
is the name that is registered for the virtual table by
the [sqlite3_create_module()] interface.

<codeblock>
   CREATE VIRTUAL TABLE tablename USING modulename;
</codeblock>

<p>One can also provide comma-separated arguments to the module following 
the module name:

<codeblock>
   CREATE VIRTUAL TABLE tablename USING modulename(arg1, arg2, ...);
</codeblock>

<p>The format of the arguments to the module is very general. Each 
<yyterm>module-argument</yyterm>
may contain keywords, string literals, identifiers, numbers, and 
punctuation. Each <yyterm>module-argument</yyterm> is passed as 
written (as text) into the
[xCreate | constructor method] of the virtual table implementation 
when the virtual 
table is created and that constructor is responsible for parsing and 
interpreting the arguments. The argument syntax is sufficiently general 
that a virtual table implementation can, if it wants to, interpret its
arguments as [column definitions] in an ordinary [CREATE TABLE] statement. 
The implementation could also impose some other interpretation on the 
arguments.

<p>Once a virtual table has been created, it can be used like any other 
table with the exceptions noted above and imposed by specific virtual
table implementations. A virtual table is destroyed using the ordinary
[DROP TABLE] syntax.

<h3>Temporary virtual tables</h3>

<p>There is no "CREATE TEMP VIRTUAL TABLE" statement.  To create a
temporary virtual table, add the "temp" schema
before the virtual table name.

<codeblock>
   CREATE VIRTUAL TABLE <b>temp.</b>tablename USING module(arg1, ...);
</codeblock>

<tcl>hd_fragment epovtab {eponymous virtual tables}  \
        {eponymous virtual table}</tcl>
<h3>Eponymous virtual tables</h3>

<p>Some virtual tables exist automatically in the "main" schema of
every database connection in which their
module is registered, even without a [CREATE VIRTUAL TABLE] statement.
Such virtual tables are called "eponymous virtual tables".
To use an eponymous virtual table, simply use the 
module name as if it were a table.
Eponymous virtual tables exist in the "main" schema only, so they will
not work if prefixed with a different schema name.

<p>An example of an eponymous virtual table is the [dbstat virtual table].
To use the dbstat virtual table as an eponymous virtual table, 
simply query against the "dbstat"
module name, as if it were an ordinary table.  (Note that SQLite
must be compiled with the [SQLITE_ENABLE_DBSTAT_VTAB] option to include
the dbstat virtual table in the build.)

<codeblock>
   SELECT * FROM dbstat;
</codeblock>

<p>A virtual table is eponymous if its [xCreate] method is the exact same
function as the [xConnect] method, or if the [xCreate] method is NULL.
The [xCreate] method is called when a virtual table is first created
using the [CREATE VIRTUAL TABLE] statement.  The [xConnect] method whenever
a database connection attaches to or reparses a schema. When these two methods
are the same, that indicates that the virtual table has no persistent
state that needs to be created and destroyed.

<tcl>hd_fragment epoonlyvtab {eponymous-only virtual table}</tcl>
<h3>Eponymous-only virtual tables</h3>
<p>If the [xCreate] method is NULL, then
[CREATE VIRTUAL TABLE] statements are prohibited for that virtual table,
and the virtual table is an "eponymous-only virtual table".
Eponymous-only virtual tables are useful as 
[table-valued functions].

<p>
Note that prior to [version 3.9.0] ([dateof:3.9.0]), 
SQLite did not check the xCreate method
for NULL before invoking it.  So if an eponymous-only virtual table is
registered with SQLite [version 3.8.11.1] ([dateof:3.8.11.1])
or earlier and a [CREATE VIRTUAL TABLE]
command is attempted against that virtual table module, a jump to a NULL
pointer will occur, resulting in a crash.

<h2>Implementation</h2>

<p>Several new C-level objects are used by the virtual table implementation:

<codeblock>
  typedef struct sqlite3_vtab sqlite3_vtab;
  typedef struct sqlite3_index_info sqlite3_index_info;
  typedef struct sqlite3_vtab_cursor sqlite3_vtab_cursor;
  typedef struct sqlite3_module sqlite3_module;
</codeblock>

<p>The [sqlite3_module] structure defines a module object used to implement
a virtual table. Think of a module as a class from which one can 
construct multiple virtual tables having similar properties. For example,
one might have a module that provides read-only access to 
comma-separated-value (CSV) files on disk. That one module can then be
used to create several virtual tables where each virtual table refers
to a different CSV file.

<p>The module structure contains methods that are invoked by SQLite to
perform various actions on the virtual table such as creating new
instances of a virtual table or destroying old ones, reading and
writing data, searching for and deleting, updating, or inserting rows. 
The module structure is explained in more detail below.

<p>Each virtual table instance is represented by an [sqlite3_vtab] structure. 
The sqlite3_vtab structure looks like this:

<codeblock>
  struct sqlite3_vtab {
    const sqlite3_module *pModule;
    int nRef;
    char *zErrMsg;
  };
</codeblock>

<p>Virtual table implementations will normally subclass this structure 
to add additional private and implementation-specific fields. 
The nRef field is used internally by the SQLite core and should not 
be altered by the virtual table implementation. The virtual table 
implementation may pass error message text to the core by putting 
an error message string in zErrMsg.
Space to hold this error message string must be obtained from an
SQLite memory allocation function such as [sqlite3_mprintf()] or
[sqlite3_malloc()].
Prior to assigning a new value to zErrMsg, the virtual table 
implementation must free any preexisting content of zErrMsg using 
[sqlite3_free()]. Failure to do this will result in a memory leak. 
The SQLite core will free and zero the content of zErrMsg when it 
delivers the error message text to the client application or when 
it destroys the virtual table. The virtual table implementation only 
needs to worry about freeing the zErrMsg content when it overwrites 
the content with a new, different error message.

<p>The [sqlite3_vtab_cursor] structure represents a pointer to a specific
row of a virtual table. This is what an sqlite3_vtab_cursor looks like:

<codeblock>
  struct sqlite3_vtab_cursor {
    sqlite3_vtab *pVtab;
  };
</codeblock>

<p>Once again, practical implementations will likely subclass this 
structure to add additional private fields.

<p>The [sqlite3_index_info] structure is used to pass information into
and out of the xBestIndex method of the module that implements a 
virtual table.

<p>Before a [CREATE VIRTUAL TABLE] statement can be run, the module 
specified in that statement must be registered with the database 
connection. This is accomplished using either of the [sqlite3_create_module()]
or [sqlite3_create_module_v2()] interfaces:

<codeblock>
  int sqlite3_create_module(
    sqlite3 *db,               /* SQLite connection to register module with */
    const char *zName,         /* Name of the module */
    const sqlite3_module *,    /* Methods for the module */
    void *                     /* Client data for xCreate/xConnect */
  );
  int sqlite3_create_module_v2(
    sqlite3 *db,               /* SQLite connection to register module with */
    const char *zName,         /* Name of the module */
    const sqlite3_module *,    /* Methods for the module */
    void *,                    /* Client data for xCreate/xConnect */
    void(*xDestroy)(void*)     /* Client data destructor function */
  );
</codeblock>

<p>The [sqlite3_create_module()] and [sqlite3_create_module_v2()]
routines associates a module name with 
an [sqlite3_module] structure and a separate client data that is specific 
to each module.  The only difference between the two create_module methods
is that the _v2 method includes an extra parameter that specifies a
destructor for client data pointer.  The module structure is what defines
the behavior of a virtual table.  The module structure looks like this:

<codeblock>  
  struct sqlite3_module {
    int iVersion;
    int (*xCreate)(sqlite3*, void *pAux,
                 int argc, char **argv,
                 sqlite3_vtab **ppVTab,
                 char **pzErr);
    int (*xConnect)(sqlite3*, void *pAux,
                 int argc, char **argv,
                 sqlite3_vtab **ppVTab,
                 char **pzErr);
    int (*xBestIndex)(sqlite3_vtab *pVTab, sqlite3_index_info*);
    int (*xDisconnect)(sqlite3_vtab *pVTab);
    int (*xDestroy)(sqlite3_vtab *pVTab);
    int (*xOpen)(sqlite3_vtab *pVTab, sqlite3_vtab_cursor **ppCursor);
    int (*xClose)(sqlite3_vtab_cursor*);
    int (*xFilter)(sqlite3_vtab_cursor*, int idxNum, const char *idxStr,
                  int argc, sqlite3_value **argv);
    int (*xNext)(sqlite3_vtab_cursor*);
    int (*xEof)(sqlite3_vtab_cursor*);
    int (*xColumn)(sqlite3_vtab_cursor*, sqlite3_context*, int);
    int (*xRowid)(sqlite3_vtab_cursor*, sqlite_int64 *pRowid);
    int (*xUpdate)(sqlite3_vtab *, int, sqlite3_value **, sqlite_int64 *);
    int (*xBegin)(sqlite3_vtab *pVTab);
    int (*xSync)(sqlite3_vtab *pVTab);
    int (*xCommit)(sqlite3_vtab *pVTab);
    int (*xRollback)(sqlite3_vtab *pVTab);
    int (*xFindFunction)(sqlite3_vtab *pVtab, int nArg, const char *zName,
                       void (**pxFunc)(sqlite3_context*,int,sqlite3_value**),
                       void **ppArg);
    int (*Rename)(sqlite3_vtab *pVtab, const char *zNew);
    /* The methods above are in version 1 of the sqlite_module object. Those 
    ** below are for version 2 and greater. */
    int (*xSavepoint)(sqlite3_vtab *pVTab, int);
    int (*xRelease)(sqlite3_vtab *pVTab, int);
    int (*xRollbackTo)(sqlite3_vtab *pVTab, int);
  };
</codeblock>

<p>The module structure defines all of the methods for each virtual 
table object. The module structure also contains the iVersion field which
defines the particular edition of the module table structure. Currently, 
iVersion is always 1, but in future releases of SQLite the module structure 
definition might be extended with additional methods and in that case 
the iVersion value will be increased.

<p>The rest of the module structure consists of methods used to implement
various features of the virtual table. Details on what each of these 
methods do are provided in the sequel.

<h2>Virtual Tables And Shared Cache</h2>

<p>Prior to SQLite [version 3.6.17] ([dateof:3.6.17]), 
the virtual table mechanism assumes 
that each [database connection] kept
its own copy of the database schema. Hence, the virtual table mechanism
could not be used in a database that has [shared cache mode] enabled. 
The [sqlite3_create_module()] interface would return an error if 
[shared cache mode] is enabled.  That restriction was relaxed
beginning with SQLite [version 3.6.17].

<h2>Creating New Virtual Table Implementations</h2>

<p>Follow these steps to create your own virtual table:

<p>
<ol>
<li> Write all necessary methods.
<li> Create an instance of the [sqlite3_module] structure containing pointers
     to all the methods from step 1.
<li> Register your [sqlite3_module] structure using one of the
     [sqlite3_create_module()] or [sqlite3_create_module_v2()] interfaces.
<li> Run a [CREATE VIRTUAL TABLE] command that specifies the new module in 
     the USING clause. 
</ol>

<p>The only really hard part is step 1. You might want to start with an 
existing virtual table implementation and modify it to suit your needs.
There are several virtual table implementations in the SQLite source tree 
(for testing purposes). You might use one of those as a guide. Locate 
these test virtual table implementations by searching 
for "sqlite3_create_module".

<p>You might also want to implement your new virtual table as a 
[sqlite3_load_extension | loadable extension].

<h1>Virtual Table Methods</h1>

<tcl>hd_fragment xcreate {sqlite3_module.xCreate} {xCreate}</tcl>
<h2>The xCreate Method</h2>

<codeblock>
  int (*xCreate)(sqlite3 *db, void *pAux,
               int argc, char **argv,
               sqlite3_vtab **ppVTab,
               char **pzErr);
</codeblock>

<p>The xCreate method is called to create a new instance of a virtual table 
in response to a [CREATE VIRTUAL TABLE] statement.
If the xCreate method is the same pointer as the [xConnect] method, then the
virtual table is an [eponymous virtual table].
If the xCreate method is omitted (if it is a NULL pointer) then the virtual 
table is an [eponymous-only virtual table].


<p>The db parameter is a pointer to the SQLite [database connection] that 
is executing the [CREATE VIRTUAL TABLE] statement. 
The pAux argument is the copy of the client data pointer that was the 
fourth argument to the [sqlite3_create_module()] or
[sqlite3_create_module_v2()] call that registered the 
[sqlite3_module | virtual table module]. 
The argv parameter is an array of argc pointers to null terminated strings. 
The first string, argv[0], is the name of the module being invoked.   The
module name is the name provided as the second argument to 
[sqlite3_create_module()] and as the argument to the USING clause of the
[CREATE VIRTUAL TABLE] statement that is running.
The second, argv[1], is the name of the database in which the new virtual table is being created. The database name is "main" for the primary database, or
"temp" for TEMP database, or the name given at the end of the [ATTACH]
statement for attached databases.  The third element of the array, argv[2], 
is the name of the new virtual table, as specified following the TABLE
keyword in the [CREATE VIRTUAL TABLE] statement.
If present, the fourth and subsequent strings in the argv[] array report 
the arguments to the module name in the [CREATE VIRTUAL TABLE] statement.

<p>The job of this method is to construct the new virtual table object
(an [sqlite3_vtab] object) and return a pointer to it in *ppVTab.

<p>As part of the task of creating a new [sqlite3_vtab] structure, this 
method <u>must</u> invoke [sqlite3_declare_vtab()] to tell the SQLite 
core about the columns and datatypes in the virtual table. 
The [sqlite3_declare_vtab()] API has the following prototype:

<codeblock>
    int sqlite3_declare_vtab(sqlite3 *db, const char *zCreateTable)
</codeblock>

<p>The first argument to [sqlite3_declare_vtab()] must be the same 
[database connection] pointer as the first parameter to this method.
The second argument to [sqlite3_declare_vtab()] must a zero-terminated 
UTF-8 string that contains a well-formed [CREATE TABLE] statement that 
defines the columns in the virtual table and their data types. 
The name of the table in this CREATE TABLE statement is ignored, 
as are all constraints. Only the column names and datatypes matter.
The CREATE TABLE statement string need not to be 
held in persistent memory.  The string can be
deallocated and/or reused as soon as the [sqlite3_declare_vtab()]
routine returns.

<p>The xCreate method need not initialize the pModule, nRef, and zErrMsg
fields of the [sqlite3_vtab] object.  The SQLite core will take care of 
that chore.

<p>The xCreate should return [SQLITE_OK] if it is successful in 
creating the new virtual table, or [SQLITE_ERROR] if it is not successful.
If not successful, the [sqlite3_vtab] structure must not be allocated. 
An error message may optionally be returned in *pzErr if unsuccessful.
Space to hold the error message string must be allocated using
an SQLite memory allocation function like 
[sqlite3_malloc()] or [sqlite3_mprintf()] as the SQLite core will
attempt to free the space using [sqlite3_free()] after the error has
been reported up to the application.

<p>
If the xCreate method is omitted (left as a NULL pointer) then the
virtual table is an [eponymous-only virtual table].  New instances of
the virtual table cannot be created using [CREATE VIRTUAL TABLE] and the
virtual table can only be used via its module name.
Note that SQLite versions prior to 3.9.0 ([dateof:3.9.0]) do not understand
eponymous-only virtual tables and will segfault if an attempt is made
to [CREATE VIRTUAL TABLE] on an eponymous-only virtual table because
the xCreate method was not checked for null.

<p>
If the xCreate method is the exact same pointer as the [xConnect] method,
that indicates that the virtual table does not need to initialize backing
store.  Such a virtual table can be used as an [eponymous virtual table]
or as a named virtual table using [CREATE VIRTUAL TABLE] or both.

<tcl>hd_fragment hiddencol {hidden column} {hidden columns}</tcl>
<h3>Hidden columns in virtual tables</h3>
<p>If a column datatype contains the special keyword "HIDDEN"
(in any combination of upper and lower case letters) then that keyword
it is omitted from the column datatype name and the column is marked 
as a hidden column internally. 
A hidden column differs from a normal column in three respects:

<p>
<ul>
<li> Hidden columns are not listed in the dataset returned by 
     "[PRAGMA table_info]",
<li> Hidden columns are not included in the expansion of a "*"
     expression in the result set of a [SELECT], and
<li> Hidden columns are not included in the implicit column-list 
     used by an [INSERT] statement that lacks an explicit column-list. 
</ul>

<p>For example, if the following SQL is passed to [sqlite3_declare_vtab()]:

<codeblock>
   CREATE TABLE x(a HIDDEN VARCHAR(12), b INTEGER, c INTEGER Hidden);
</codeblock>

<p>Then the virtual table would be created with two hidden columns,
and with datatypes of "VARCHAR(12)" and "INTEGER".

<p>An example use of hidden columns can be seen in the [FTS3] virtual 
table implementation, where every FTS virtual table
contains an [FTS hidden column] that is used to pass information from the
virtual table into [FTS auxiliary functions] and to the [FTS MATCH] operator.

<tcl>hd_fragment tabfunc2 {table-valued functions} \
         {table-valued function}</tcl>
<h3>Table-valued functions</h3>

<p>A [virtual table] that contains [hidden columns] can be used like
a table-valued function in the FROM clause of a [SELECT] statement.
The arguments to the table-valued function become constraints on 
the HIDDEN columns of the virtual table.

<p>For example, the "generate_series" extension (located in the
[http://www.sqlite.org/src/artifact?ci=trunk&filename=ext/misc/series.c|ext/misc/series.c]
file in the [http://www.sqlite.org/src/tree?ci=trunk|source tree])
implements an [eponymous virtual table] with the following schema:

<codeblock>
CREATE TABLE generate_series(
  value,
  start HIDDEN,
  stop HIDDEN,
  step HIDDEN
);
</codeblock>

<p>The [sqlite3_module.xBestIndex] method in the implementation of this
table checks for equality constraints against the HIDDEN columns, and uses
those as input parameters to determine the range of integer "value" outputs
to generate.  Reasonable defaults are used for any unconstrained columns.
For example, to list all integers between 5 and 50:

<codeblock>
SELECT value FROM generate_series(5,50);
</codeblock>

<p>The previous query is equivalent to the following:

<codeblock>
SELECT value FROM generate_series WHERE start=5 AND stop=50;
</codeblock>

<p>Arguments on the virtual table name are matched to [hidden columns]
in order.  The number of arguments can be less than the
number of hidden columns, in which case the latter hidden columns are
unconstrained.  However, an error results if there are more arguments
than there are hidden columns in the virtual table.

<tcl>hd_fragment worid {WITHOUT ROWID virtual tables} \
{WITHOUT ROWID virtual table}</tcl>
<h3> WITHOUT ROWID Virtual Tables </h3>

<p>Beginning with SQLite [version 3.14.0] ([dateof:3.14]), 
the CREATE TABLE statement that
is passed into [sqlite3_declare_vtab()] may contain a [WITHOUT ROWID] clause.
This is useful for cases where the virtual table rows 
cannot easily be mapped into unique integers.  A CREATE TABLE
statement that includes WITHOUT ROWID must define one or more columns as
the PRIMARY KEY.  Every column of the PRIMARY KEY must individually be
NOT NULL and all columns for each row must be collectively unique.

<p>Note that SQLite does not enforce the PRIMARY KEY for a WITHOUT ROWID
virtual table.  Enforcement is the responsibility of the underlying
virtual table implementation.  But SQLite does assume that the PRIMARY KEY
constraint is valid - that the identified columns really are UNIQUE and
NOT NULL - and it uses that assumption to optimize queries against the
virtual table.

<p>The rowid column is not accessible on a
WITHOUT ROWID virtual table (of course).

<p>The [xUpdate] method was originally designed around having a
[ROWID] as a single value.  The [xUpdate] method has been expanded to
accommodate an arbitrary PRIMARY KEY in place of the ROWID, but the
PRIMARY KEY must still be only one column.  For this reason, SQLite
will reject any WITHOUT ROWID virtual table that has more than one
PRIMARY KEY column and a non-NULL xUpdate method.

<tcl>############################################################# xConnect
hd_fragment xconnect {sqlite3_module.xConnect} {xConnect}</tcl>
<h2>The xConnect Method</h2>

<codeblock>
  int (*xConnect)(sqlite3*, void *pAux,
               int argc, char **argv,
               sqlite3_vtab **ppVTab,
               char **pzErr);
</codeblock>

<p>The xConnect method is very similar to [xCreate]. 
It has the same parameters and constructs a new [sqlite3_vtab] structure 
just like xCreate. 
And it must also call [sqlite3_declare_vtab()] like xCreate.

<p>The difference is that xConnect is called to establish a new 
connection to an existing virtual table whereas xCreate is called 
to create a new virtual table from scratch.

<p>The xCreate and xConnect methods are only different when the
virtual table has some kind of backing store that must be initialized 
the first time the virtual table is created. The xCreate method creates 
and initializes the backing store. The xConnect method just connects 
to an existing backing store.  When xCreate and xConnect are the same,
the table is an [eponymous virtual table].

<p>As an example, consider a virtual table implementation that 
provides read-only access to existing comma-separated-value (CSV)
files on disk. There is no backing store that needs to be created 
or initialized for such a virtual table (since the CSV files already 
exist on disk) so the xCreate and xConnect methods will be identical 
for that module.

<p>Another example is a virtual table that implements a full-text index. 
The xCreate method must create and initialize data structures to hold 
the dictionary and posting lists for that index. The xConnect method,
on the other hand, only has to locate and use an existing dictionary 
and posting lists that were created by a prior xCreate call.

<p>The xConnect method must return [SQLITE_OK] if it is successful 
in creating the new virtual table, or [SQLITE_ERROR] if it is not 
successful. If not successful, the [sqlite3_vtab] structure must not be 
allocated. An error message may optionally be returned in *pzErr if 
unsuccessful. 
Space to hold the error message string must be allocated using
an SQLite memory allocation function like 
[sqlite3_malloc()] or [sqlite3_mprintf()] as the SQLite core will
attempt to free the space using [sqlite3_free()] after the error has
been reported up to the application.

<p>The xConnect method is required for every virtual table implementation, 
though the [xCreate] and xConnect pointers of the [sqlite3_module] object
may point to the same function if the virtual table does not need to
initialize backing store.

<tcl>############################################################ xBestIndex
hd_fragment xbestindex {sqlite3_module.xBestIndex} {xBestIndex}</tcl>
<h2>The xBestIndex Method</h2>

<p>SQLite uses the xBestIndex method of a virtual table module to determine
the best way to access the virtual table. 
The xBestIndex method has a prototype like this:

<codeblock>
  int (*xBestIndex)(sqlite3_vtab *pVTab, sqlite3_index_info*);
</codeblock>

<p>The SQLite core communicates with the xBestIndex method by filling 
in certain fields of the [sqlite3_index_info] structure and passing a 
pointer to that structure into xBestIndex as the second parameter. 
The xBestIndex method fills out other fields of this structure which
forms the reply. The [sqlite3_index_info] structure looks like this:

<codeblock>
  struct sqlite3_index_info {
    /* Inputs */
    const int nConstraint;     /* Number of entries in aConstraint */
    const struct sqlite3_index_constraint {
       int iColumn;              /* Column constrained.  -1 for ROWID */
       unsigned char op;         /* Constraint operator */
       unsigned char usable;     /* True if this constraint is usable */
       int iTermOffset;          /* Used internally - xBestIndex should ignore */
    } *const aConstraint;      /* Table of WHERE clause constraints */
    const int nOrderBy;        /* Number of terms in the ORDER BY clause */
    const struct sqlite3_index_orderby {
       int iColumn;              /* Column number */
       unsigned char desc;       /* True for DESC.  False for ASC. */
    } *const aOrderBy;         /* The ORDER BY clause */

    /* Outputs */
    struct sqlite3_index_constraint_usage {
      int argvIndex;           /* if >0, constraint is part of argv to xFilter */
      unsigned char omit;      /* Do not code a test for this constraint */
    } *const aConstraintUsage;
    int idxNum;                /* Number used to identify the index */
    char *idxStr;              /* String, possibly obtained from sqlite3_malloc */
    int needToFreeIdxStr;      /* Free idxStr using sqlite3_free() if true */
    int orderByConsumed;       /* True if output is already ordered */
    double estimatedCost;      /* Estimated cost of using this index */
    <b>/* Fields below are only available in SQLite 3.8.2 and later */</b>
    sqlite3_int64 estimatedRows;    /* Estimated number of rows returned */
    <b>/* Fields below are only available in SQLite 3.9.0 and later */</b>
    int idxFlags;              /* Mask of SQLITE_INDEX_SCAN_* flags */
    <b>/* Fields below are only available in SQLite 3.10.0 and later */</b>
    sqlite3_uint64 colUsed;    /* Input: Mask of columns used by statement */
  };
</codeblock>

<p>Note the warnings on the "estimatedRows", "idxFlags", and colUsed fields.
These fields were added with SQLite versions 3.8.2, 3.9.0, and 3.10.0, respectively. 
Any extension that reads or writes these fields must first check that the 
version of the SQLite library in use is greater than or equal to appropriate
version - perhaps comparing the value returned from [sqlite3_libversion_number()]
against constants 3008002, 3009000, and/or 3010000. The result of attempting 
to access these fields in an sqlite3_index_info structure created by an 
older version of SQLite are undefined.

<p>In addition, there are some defined constants:

<codeblock>
  #define SQLITE_INDEX_CONSTRAINT_EQ         2
  #define SQLITE_INDEX_CONSTRAINT_GT         4
  #define SQLITE_INDEX_CONSTRAINT_LE         8
  #define SQLITE_INDEX_CONSTRAINT_LT        16
  #define SQLITE_INDEX_CONSTRAINT_GE        32
  #define SQLITE_INDEX_CONSTRAINT_MATCH     64
  #define SQLITE_INDEX_CONSTRAINT_LIKE      65  /* 3.10.0 and later */
  #define SQLITE_INDEX_CONSTRAINT_GLOB      66  /* 3.10.0 and later */
  #define SQLITE_INDEX_CONSTRAINT_REGEXP    67  /* 3.10.0 and later */
  #define SQLITE_INDEX_CONSTRAINT_NE        68  /* 3.21.0 and later */
  #define SQLITE_INDEX_CONSTRAINT_ISNOT     69  /* 3.21.0 and later */
  #define SQLITE_INDEX_CONSTRAINT_ISNOTNULL 70  /* 3.21.0 and later */
  #define SQLITE_INDEX_CONSTRAINT_ISNULL    71  /* 3.21.0 and later */
  #define SQLITE_INDEX_CONSTRAINT_IS        72  /* 3.21.0 and later */
  #define SQLITE_INDEX_SCAN_UNIQUE           1  /* Scan visits at most 1 row */
</codeblock>

<p>The SQLite core calls the xBestIndex method when it is compiling a query
that involves a virtual table. In other words, SQLite calls this method 
when it is running [sqlite3_prepare()] or the equivalent. 
By calling this method, the 
SQLite core is saying to the virtual table that it needs to access 
some subset of the rows in the virtual table and it wants to know the
most efficient way to do that access. The xBestIndex method replies 
with information that the SQLite core can then use to conduct an 
efficient search of the virtual table.

<p>While compiling a single SQL query, the SQLite core might call 
xBestIndex multiple times with different settings in [sqlite3_index_info].
The SQLite core will then select the combination that appears to 
give the best performance.

<p>Before calling this method, the SQLite core initializes an instance 
of the [sqlite3_index_info] structure with information about the
query that it is currently trying to process. This information 
derives mainly from the WHERE clause and ORDER BY or GROUP BY clauses 
of the query, but also from any ON or USING clauses if the query is a 
join. The information that the SQLite core provides to the xBestIndex 
method is held in the part of the structure that is marked as "Inputs". 
The "Outputs" section is initialized to zero.

<p>The information in the [sqlite3_index_info] structure is ephemeral
and may be overwritten or deallocated as soon as the xBestIndex method
returns.  If the xBestIndex method needs to remember any part of the
[sqlite3_index_info] structure, it should make a copy.  Care must be
take to store the copy in a place where it will be deallocated, such
as in the idxStr field with needToFreeIdxStr set to 1.

<p>Note that xBestIndex will always be called before [xFilter], since
the idxNum and idxStr outputs from xBestIndex are required inputs to
xFilter.  However, there is no guarantee that xFilter will be called
following a successful xBestIndex.  

<p>The xBestIndex method is required for every virtual table implementation.

<h3>Inputs</h3>

<p>The main thing that the SQLite core is trying to communicate to 
the virtual table is the constraints that are available to limit 
the number of rows that need to be searched. The aConstraint[] array 
contains one entry for each constraint. There will be exactly 
nConstraint entries in that array.

<p>Each constraint will correspond to a term in the WHERE clause
or in a USING or ON clause that is of the form

<blockquote>
     column  OP  EXPR
</blockquote>

<p>Where "column" is a column in the virtual table, OP is an operator 
like "=" or "&lt;", and EXPR is an arbitrary expression. So, for example,
if the WHERE clause contained a term like this:

<codeblock>
     a = 5
</codeblock>

<p>Then one of the constraints would be on the "a" column with 
operator "=" and an expression of "5". Constraints need not have a
literal representation of the WHERE clause. The query optimizer might
make transformations to the 
WHERE clause in order to extract as many constraints 
as it can. So, for example, if the WHERE clause contained something 
like this:

<codeblock>
     x BETWEEN 10 AND 100 AND 999&gt;y
</codeblock>

<p>The query optimizer might translate this into three separate constraints:

<codeblock>
     x &gt;= 10
     x &lt;= 100
     y &lt; 999
</codeblock>

<p>For each constraint, the aConstraint[].iColumn field indicates which 
column appears on the left-hand side of the constraint.
The first column of the virtual table is column 0. 
The rowid of the virtual table is column -1. 
The aConstraint[].op field indicates which operator is used. 
The SQLITE_INDEX_CONSTRAINT_* constants map integer constants 
into operator values.
Columns occur in the order they were defined by the call to
[sqlite3_declare_vtab()] in the [xCreate] or [xConnect] method.
Hidden columns are counted when determining the column index.

<p>The aConstraint[] array contains information about all constraints 
that apply to the virtual table. But some of the constraints might
not be usable because of the way tables are ordered in a join. 
The xBestIndex method must therefore only consider constraints 
that have an aConstraint[].usable flag which is true.

<p>In addition to WHERE clause constraints, the SQLite core also 
tells the xBestIndex method about the ORDER BY clause. 
(In an aggregate query, the SQLite core might put in GROUP BY clause 
information in place of the ORDER BY clause information, but this fact
should not make any difference to the xBestIndex method.) 
If all terms of the ORDER BY clause are columns in the virtual table, 
then nOrderBy will be the number of terms in the ORDER BY clause 
and the aOrderBy[] array will identify the column for each term 
in the order by clause and whether or not that column is ASC or DESC.

<tcl>hd_fragment colUsed {colUsed field}</tcl>
<p>In SQLite [version 3.10.0] ([dateof:3.10.0]) and later, 
the colUsed field is available
to indicate which fields of the virtual table are actually used by the
statement being prepared.  If the lowest bit of colUsed is set, that
means that the first column is used.  The second lowest bit corresponds
to the second column.  And so forth.  If the most significant bit of
colUsed is set, that means that one or more columns other than the 
first 63 columns are used.  If column usage information is needed by the
[xFilter] method, then the required bits must be encoded into either
the idxNum or idxStr output fields.

<h3>Outputs</h3>

<p>Given all of the information above, the job of the xBestIndex 
method it to figure out the best way to search the virtual table.

<p>The xBestIndex method fills the idxNum and idxStr fields with 
information that communicates an indexing strategy to the [xFilter] 
method. The information in idxNum and idxStr is arbitrary as far 
as the SQLite core is concerned. The SQLite core just copies the 
information through to the [xFilter] method. Any desired meaning can 
be assigned to idxNum and idxStr as long as xBestIndex and xFilter 
agree on what that meaning is.

<p>The idxStr value may be a string obtained from an SQLite
memory allocation function such as [sqlite3_mprintf()]. 
If this is the case, then the needToFreeIdxStr flag must be set to 
true so that the SQLite core will know to call [sqlite3_free()] on 
that string when it has finished with it, and thus avoid a memory leak.

<p>If the virtual table will output rows in the order specified by 
the ORDER BY clause, then the orderByConsumed flag may be set to 
true. If the output is not automatically in the correct order 
then orderByConsumed must be left in its default false setting. 
This will indicate to the SQLite core that it will need to do a 
separate sorting pass over the data after it comes out of the virtual table.

<p>The estimatedCost field should be set to the estimated number
of disk access operations required to execute this query against 
the virtual table. The SQLite core will often call xBestIndex 
multiple times with different constraints, obtain multiple cost
estimates, then choose the query plan that gives the lowest estimate.

<p>If the current version of SQLite is 3.8.2 or greater, the estimatedRows
field may be set to an estimate of the number of rows returned by the
proposed query plan. If this value is not explicitly set, the default 
estimate of 25 rows is used.

<p>If the current version of SQLite is 3.9.0 or greater, the idxFlags field
may be set to SQLITE_INDEX_SCAN_UNIQUE to indicate that the virtual table
will return only zero or one rows given the input constraints.  Additional
bits of the idxFlags field might be understood in later versions of SQLite.

<p>The aConstraintUsage[] array contains one element for each of 
the nConstraint constraints in the inputs section of the 
[sqlite3_index_info] structure. 
The aConstraintUsage[] array is used by xBestIndex to tell the 
core how it is using the constraints.

<p>The xBestIndex method may set aConstraintUsage[].argvIndex 
entries to values greater than zero. 
Exactly one entry should be set to 1, another to 2, another to 3, 
and so forth up to as many or as few as the xBestIndex method wants. 
The EXPR of the corresponding constraints will then be passed 
in as the argv[] parameters to xFilter.

<p>For example, if the aConstraint[3].argvIndex is set to 1, then 
when xFilter is called, the argv[0] passed to xFilter will have 
the EXPR value of the aConstraint[3] constraint.

<p>By default, the SQLite core double checks all constraints on 
each row of the virtual table that it receives. If such a check 
is redundant, the xBestFilter method can suppress that double-check by 
setting aConstraintUsage[].omit.

<tcl>hd_fragment xdisconnect {sqlite3_module.xDisconnect} {xDisconnect}</tcl>
<h2>The xDisconnect Method</h2>

<codeblock>
  int (*xDisconnect)(sqlite3_vtab *pVTab);
</codeblock>

<p>This method releases a connection to a virtual table. 
Only the [sqlite3_vtab] object is destroyed.
The virtual table is not destroyed and any backing store 
associated with the virtual table persists. 

This method undoes the work of [xConnect].

<p>This method is a destructor for a connection to the virtual table.
Contrast this method with [xDestroy].  The xDestroy is a destructor
for the entire virtual table.

<p>The xDisconnect method is required for every virtual table implementation,
though it is acceptable for the xDisconnect and [xDestroy] methods to be
the same function if that makes sense for the particular virtual table.

<tcl>########################################################## xDestroy
hd_fragment {sqlite3_module.xDestroy} {xDestroy}</tcl>
<h2>The xDestroy Method</h2>

<codeblock>
  int (*xDestroy)(sqlite3_vtab *pVTab);
</codeblock>

<p>This method releases a connection to a virtual table, just like 
the [xDisconnect] method, and it also destroys the underlying 
table implementation. This method undoes the work of [xCreate].

<p>The [xDisconnect] method is called whenever a database connection
that uses a virtual table is closed. The xDestroy method is only 
called when a [DROP TABLE] statement is executed against the virtual table.

<p>The xDestroy method is required for every virtual table implementation,
though it is acceptable for the [xDisconnect] and xDestroy methods to be
the same function if that makes sense for the particular virtual table.

<tcl>########################################################## xOpen
hd_fragment xopen {sqlite3_module.xOpen}</tcl>
<h2>The xOpen Method</h2>

<codeblock>
  int (*xOpen)(sqlite3_vtab *pVTab, sqlite3_vtab_cursor **ppCursor);
</codeblock>

<p>The xOpen method creates a new cursor used for accessing (read and/or
writing) a virtual table.  A successful invocation of this method 
will allocate the memory for the [sqlite3_vtab_cursor] (or a subclass),
initialize the new object, and make *ppCursor point to the new object.
The successful call then returns [SQLITE_OK].

<p>For every successful call to this method, the SQLite core will
later invoke the [sqlite3_module.xClose | xClose] method to destroy 
the allocated cursor.

<p>The xOpen method need not initialize the pVtab field of the
[sqlite3_vtab_cursor] structure.  The SQLite core will take care
of that chore automatically.

<p>A virtual table implementation must be able to support an arbitrary
number of simultaneously open cursors.

<p>When initially opened, the cursor is in an undefined state.
The SQLite core will invoke the [xFilter] method
on the cursor prior to any attempt to position or read from the cursor.

<p>The xOpen method is required for every virtual table implementation.

<tcl>############################################################### xClose
hd_fragment xclose {sqlite3_module.xClose}</tcl>
<h2>The xClose Method</h2>

<codeblock>
  int (*xClose)(sqlite3_vtab_cursor*);
</codeblock>

<p>The xClose method closes a cursor previously opened by 
[sqlite3_module.xOpen | xOpen]. 
The SQLite core will always call xClose once for each cursor opened 
using xOpen.

<p>This method must release all resources allocated by the
corresponding xOpen call. The routine will not be called again even if it
returns an error.  The SQLite core will not use the
[sqlite3_vtab_cursor] again after it has been closed.

<p>The xClose method is required for every virtual table implementation.

<tcl>############################################################## xEof
hd_fragment xeof {sqlite3_module.xEof} {xEof}</tcl>
<h2>The xEof Method</h2>

<codeblock>
  int (*xEof)(sqlite3_vtab_cursor*);
</codeblock>

<p>The xEof method must return false (zero) if the specified cursor 
currently points to a valid row of data, or true (non-zero) otherwise. 
This method is called by the SQL engine immediately after each 
[xFilter] and [xNext] invocation.

<p>The xEof method is required for every virtual table implementation.

<tcl>############################################################## xFilter
hd_fragment xfilter {sqlite3_module.xFilter} {xFilter}</tcl>
<h2>The xFilter Method</h2>

<codeblock>
  int (*xFilter)(sqlite3_vtab_cursor*, int idxNum, const char *idxStr,
                int argc, sqlite3_value **argv);
</codeblock>

<p>This method begins a search of a virtual table. 
The first argument is a cursor opened by [sqlite3_module.xOpen | xOpen]. 
The next two arguments define a particular search index previously 
chosen by [xBestIndex]. The specific meanings of idxNum and idxStr 
are unimportant as long as xFilter and xBestIndex agree on what 
that meaning is.

<p>The xBestIndex function may have requested the values of 
certain expressions using the aConstraintUsage[].argvIndex values 
of the [sqlite3_index_info] structure. 
Those values are passed to xFilter using the argc and argv parameters.

<p>If the virtual table contains one or more rows that match the
search criteria, then the cursor must be left point at the first row.
Subsequent calls to [xEof] must return false (zero).
If there are no rows match, then the cursor must be left in a state 
that will cause the [xEof] to return true (non-zero).
The SQLite engine will use
the [xColumn] and [xRowid] methods to access that row content.
The [xNext] method will be used to advance to the next row.

<p>This method must return [SQLITE_OK] if successful, or an sqlite 
[error code] if an error occurs.

<p>The xFilter method is required for every virtual table implementation.

<tcl>############################################################### xNext
hd_fragment xnext {sqlite3_module.xNext} {xNext}</tcl>
<h2>The xNext Method</h2>

<codeblock>
  int (*xNext)(sqlite3_vtab_cursor*);
</codeblock>

<p>The xNext method advances a [sqlite3_vtab_cursor | virtual table cursor]
to the next row of a result set initiated by [xFilter]. 
If the cursor is already pointing at the last row when this 
routine is called, then the cursor no longer points to valid 
data and a subsequent call to the [xEof] method must return true (non-zero). 
If the cursor is successfully advanced to another row of content, then
subsequent calls to [xEof] must return false (zero).

<p>This method must return [SQLITE_OK] if successful, or an sqlite 
[error code] if an error occurs.

<p>The xNext method is required for every virtual table implementation.

<tcl>############################################################## xColumn
hd_fragment xcolumn {sqlite3_module.xColumn} {xColumn}</tcl>
<h2>The xColumn Method</h2>

<codeblock>
  int (*xColumn)(sqlite3_vtab_cursor*, sqlite3_context*, int N);
</codeblock>

<p>The SQLite core invokes this method in order to find the value for 
the N-th column of the current row. N is zero-based so the first column 
is numbered 0. 
The xColumn method may return its result back to SQLite using one of the
following interface:

<p>
<ul>
<li> [sqlite3_result_blob()]
<li> [sqlite3_result_double()]
<li> [sqlite3_result_int()]
<li> [sqlite3_result_int64()]
<li> [sqlite3_result_null()]
<li> [sqlite3_result_text()]
<li> [sqlite3_result_text16()]
<li> [sqlite3_result_text16le()]
<li> [sqlite3_result_text16be()]
<li> [sqlite3_result_zeroblob()]
</ul>
</p>

<p>If the xColumn method implementation calls none of the functions above,
then the value of the column defaults to an SQL NULL.

<p>To raise an error, the xColumn method should use one of the result_text() 
methods to set the error message text, then return an appropriate
[error code].  The xColumn method must return [SQLITE_OK] on success.

<p>The xColumn method is required for every virtual table implementation.

<tcl>############################################################# xRowid
hd_fragment xrowid {sqlite3_module.xRowid} {xRowid}</tcl>
<h2>The xRowid Method</h2>

<codeblock>
  int (*xRowid)(sqlite3_vtab_cursor *pCur, sqlite_int64 *pRowid);
</codeblock>

<p>A successful invocation of this method will cause *pRowid to be
filled with the [rowid] of row that the
[sqlite3_vtab_cursor | virtual table cursor] pCur is currently pointing at.
This method returns [SQLITE_OK] on success.
It returns an appropriate [error code] on failure.</p>

<p>The xRowid method is required for every virtual table implementation.

<tcl>############################################################# xUpdate
hd_fragment xupdate {sqlite3_module.xUpdate} {xUpdate}</tcl>
<h2>The xUpdate Method</h2>

<codeblock>
  int (*xUpdate)(
    sqlite3_vtab *pVTab,
    int argc,
    sqlite3_value **argv,
    sqlite_int64 *pRowid
  );
</codeblock>

<p>All changes to a virtual table are made using the xUpdate method.
This one method can be used to insert, delete, or update.

<p>The argc parameter specifies the number of entries in the argv array. 
The value of argc will be 1 for a pure delete operation or N+2 for an insert
or replace or update where N is the number of columns in the table.  
In the previous sentence, N includes any hidden columns.

<p>Every argv entry will have a non-NULL value in C but may contain the 
SQL value NULL.  In other words, it is always true that
<tt>argv&#91;i&#93;!=0</tt> for <b>i</b> between 0 and <tt>argc-1</tt>.
However, it might be the case that
<tt>sqlite3_value_type(argv&#91;i&#93;)==SQLITE_NULL</tt>.

<p>The argv[0] parameter is the [rowid] of a row in the virtual table 
to be deleted. If argv[0] is an SQL NULL, then no deletion occurs.

<p>The argv[1] parameter is the rowid of a new row to be inserted 
into the virtual table. If argv[1] is an SQL NULL, then the implementation 
must choose a rowid for the newly inserted row. Subsequent argv[] 
entries contain values of the columns of the virtual table, in the 
order that the columns were declared. The number of columns will
match the table declaration that the [xConnect] or [xCreate] method made 
using the [sqlite3_declare_vtab()] call.  All hidden columns are included.

<p>When doing an insert without a rowid (argc>1, argv[1] is an SQL NULL),
on a virtual table that uses ROWID (but not on a [WITHOUT ROWID virtual table],
the implementation must set *pRowid to the rowid of the newly inserted row; 
this will become the value returned by the [sqlite3_last_insert_rowid()]
function. Setting this value in all the other cases is a harmless no-op;
the SQLite engine ignores the *pRowid return value if argc==1 or 
argv[1] is not an SQL NULL.

<p>Each call to xUpdate will fall into one of cases shown below.
Not that references to <b>argv&#91;i&#93</b> mean the SQL value
held within the argv&#91;i&#93; object, not the argv&#91;i&#93;
object itself.

<blockquote>
<dl>
<dt><b>argc = 1 <br> argv[0] &ne; NULL</b>
<dd><p>
The single row with rowid or PRIMARY KEY equal to argv[0] is deleted. 
No insert occurs.

<dt><b>argc &gt; 1 <br> argv[0] = NULL</b>
<dd><p>
A new row is inserted with column values taken from
argv[2] and following.  In a rowid virtual table, if argv[1] is an SQL NULL,
then a new unique rowid is generated automatically.  The argv[1] will be NULL
for a [WITHOUT ROWID virtual table], in which case the implementation should
take the PRIMARY KEY value from the appropriate column in argv[2] and following.

<dt><b>argc &gt; 1 <br> argv[0] &ne; NULL <br> argv[0] = argv[1]</b>
<dd><p>
The row with rowid or PRIMARY KEY argv[0] is updated with new values 
in argv[2] and following parameters.

<dt><b>argc &gt; 1 <br> argv[0] &ne; NULL <br> argv[0] &ne; argv[1]</b>
<dd><p> The row with rowid or PRIMARY KEY argv[0] is updated with 
the rowid or PRIMARY KEY in argv[1] 
and new values in argv[2] and following parameters. This will occur 
when an SQL statement updates a rowid, as in the statement:
<blockquote>
   [UPDATE] table SET rowid=rowid+1 WHERE ...; 
</blockquote>
</dl>
</blockquote>

<p>The xUpdate method must return [SQLITE_OK] if and only if it is
successful.  If a failure occurs, the xUpdate must return an appropriate
[error code].  On a failure, the pVTab->zErrMsg element may optionally
be replaced with error message text stored in memory allocated from SQLite 
using functions such as [sqlite3_mprintf()] or [sqlite3_malloc()].

<p>If the xUpdate method violates some constraint of the virtual table
(including, but not limited to, attempting to store a value of the wrong 
datatype, attempting to store a value that is too
large or too small, or attempting to change a read-only value) then the
xUpdate must fail with an appropriate [error code].

<p>If the xUpdate method is performing an UPDATE, then
[sqlite3_value_nochange(X)] can be used to discover which columns
of the virtual table were actually modified by the UPDATE
statement.  The [sqlite3_value_nochange(X)] interface returns
true for columns that do not change.
On every UPDATE, SQLite will first invoke
[xColumn] separately for each unchanging column in the table to 
obtain the value for that column.  The [xColumn] method can
check to see if the column is unchanged at the SQL level
by invoking [sqlite3_vtab_nochange()].  If [xColumn] sees that
the column is not being modified, it should return without setting 
a result using one of the [sqlite3_result_int|sqlite3_result_xxxxx()]
interfaces.  Only in that case [sqlite3_value_nochange()] will be
true within the xUpdate method.  If [xColumn] does
invoke one or more [sqlite3_result_int|sqlite3_result_xxxxx()]
interfaces, then SQLite understands that as a change in the value
of the column and the [sqlite3_value_nochange()] call for that
column within xUpdate will return false.

<p>There might be one or more [sqlite3_vtab_cursor] objects open and in use 
on the virtual table instance and perhaps even on the row of the virtual
table when the xUpdate method is invoked.  The implementation of
xUpdate must be prepared for attempts to delete or modify rows of the table
out from other existing cursors.  If the virtual table cannot accommodate
such changes, the xUpdate method must return an [error code].

<p>The xUpdate method is optional.
If the xUpdate pointer in the [sqlite3_module] for a virtual table
is a NULL pointer, then the virtual table is read-only.


<tcl>########################################################## xFindFunction
hd_fragment xfindfunction {sqlite3_module.xFindFunction} {xFindFunction}</tcl>
<h2>The xFindFunction Method</h2>

<codeblock>
  int (*xFindFunction)(
    sqlite3_vtab *pVtab,
    int nArg,
    const char *zName,
    void (**pxFunc)(sqlite3_context*,int,sqlite3_value**),
    void **ppArg
  );
</codeblock>

<p>This method is called during [sqlite3_prepare()] to give the virtual
table implementation an opportunity to overload functions. 
This method may be set to NULL in which case no overloading occurs.

<p>When a function uses a column from a virtual table as its first 
argument, this method is called to see if the virtual table would 
like to overload the function. The first three parameters are inputs: 
the virtual table, the number of arguments to the function, and the 
name of the function. If no overloading is desired, this method
returns 0. To overload the function, this method writes the new 
function implementation into *pxFunc and writes user data into *ppArg 
and returns 1.

<p>Note that infix functions ([LIKE], [GLOB], [REGEXP], and [MATCH]) reverse 
the order of their arguments. So "like(A,B)" is equivalent to "B like A". 
For the form "B like A" the B term is considered the first argument 
to the function. But for "like(A,B)" the A term is considered the 
first argument.

<p>The function pointer returned by this routine must be valid for
the lifetime of the [sqlite3_vtab] object given in the first parameter.

<tcl>############################################################ xBegin
hd_fragment xBegin {sqlite3_module.xBegin} {xBegin}</tcl>
<h2>The xBegin Method</h2>

<codeblock>
  int (*xBegin)(sqlite3_vtab *pVTab);
</codeblock>

<p>This method begins a transaction on a virtual table.
This is method is optional.  The xBegin pointer of [sqlite3_module]
may be NULL.

<p>This method is always followed by one call to either the
[xCommit] or [xRollback] method.  Virtual table transactions do
not nest, so the xBegin method will not be invoked more than once
on a single virtual table
without an intervening call to either [xCommit] or [xRollback].
Multiple calls to other methods can and likely will occur in between
the xBegin and the corresponding [xCommit] or [xRollback].

<tcl>############################################################ xSync
hd_fragment xsync {sqlite3_module.xSync}</tcl>
<h2>The xSync Method</h2>

<codeblock>
  int (*xSync)(sqlite3_vtab *pVTab);
</codeblock>


<p>This method signals the start of a two-phase commit on a virtual
table.
This is method is optional.  The xSync pointer of [sqlite3_module]
may be NULL.

<p>This method is only invoked after call to the [xBegin] method and
prior to an [xCommit] or [xRollback].  In order to implement two-phase
commit, the xSync method on all virtual tables is invoked prior to
invoking the [xCommit] method on any virtual table.  If any of the 
xSync methods fail, the entire transaction is rolled back.

<tcl>########################################################### xCommit
hd_fragment xcommit {sqlite3_module.xCommit} {xCommit}</tcl>
<h2>The xCommit Method</h2>

<codeblock>
  int (*xCommit)(sqlite3_vtab *pVTab);
</codeblock>

<p>This method causes a virtual table transaction to commit.
This is method is optional.  The xCommit pointer of [sqlite3_module]
may be NULL.

<p>A call to this method always follows a prior call to [xBegin] and
[sqlite3_module.xSync|xSync].


<tcl>############################################################## xRollback
hd_fragment xrollback {sqlite3_module.xRollback} {xRollback}</tcl>
<h2>The xRollback Method</h2>

<codeblock>
  int (*xRollback)(sqlite3_vtab *pVTab);
</codeblock>

<p>This method causes a virtual table transaction to rollback.
This is method is optional.  The xRollback pointer of [sqlite3_module]
may be NULL.

<p>A call to this method always follows a prior call to [xBegin].


<tcl>############################################################# xRename
hd_fragment xrename {sqlite3_module.xRename} {xRename}</tcl>
<h2>The xRename Method</h2>

<codeblock>
  int (*xRename)(sqlite3_vtab *pVtab, const char *zNew);
</codeblock>

<p>This method provides notification that the virtual table implementation
that the virtual table will be given a new name. 
If this method returns [SQLITE_OK] then SQLite renames the table.
If this method returns an [error code] then the renaming is prevented.

<p>The xRename method is required for every virtual table implementation.

<tcl>############################################################# xSavepoint
hd_fragment xsavepoint {sqlite3_module.xSavepoint} {xSavepoint}\
  xRelease xRollbackTo</tcl>
<h2>The xSavepoint, xRelease, and xRollbackTo Methods</h2>

<codeblock>
  int (*xSavepoint)(sqlite3_vtab *pVtab, int);
  int (*xRelease)(sqlite3_vtab *pVtab, int);
  int (*xRollbackTo)(sqlite3_vtab *pVtab, int);
</codeblock>

<p>
These methods provide the virtual table implementation an opportunity to
implement nested transactions.  They are always optional and will only be
called in SQLite [version 3.7.7] ([dateof:3.7.7]) and later.
</p>

<p>
^When xSavepoint(X,N) is invoked, that is a signal to the virtual table X
that it should save its current state as savepoint N.  
^A subsequent call
to xRollbackTo(X,R) means that the state of the virtual table should return
to what it was when xSavepoint(X,R) was last called.  
^The call
to xRollbackTo(X,R) will invalidate all savepoints with N>R; none of the
invalided savepoints will be rolled back or released without first
being reinitialized by a call to xSavepoint().  
^A call to xRelease(X,M) invalidates all savepoints where N>=M.
</p>

<p>
^None of the xSavepoint(), xRelease(), or xRollbackTo() methods will ever
be called except in between calls to xBegin() and 
either xCommit() or xRollback().
</p>
