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Comment:Add documentation for the STMT virtual table
Timelines: family | ancestors | descendants | both | trunk
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SHA3-256: 3c1197562c4f74659df8efe03e1aadce0a56bfb0a666326715a83c8a71a92da0
User & Date: drh 2017-06-29 18:17:08
Context
2017-06-29
18:27
Mention the -withoutnulls option to the "db eval" method in the TCL interface documentation. check-in: b1785f3962 user: drh tags: trunk
18:17
Add documentation for the STMT virtual table check-in: 3c1197562c user: drh tags: trunk
17:33
Mention the sqlite3_prepare_v3() enhancement in the change log. check-in: 9421ab387b user: drh tags: trunk
Changes
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Changes to pages/changes.in.

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  set aChng($nChng) [list $date $desc $options]
  set xrefChng($date) $nChng
  incr nChng
}

chng {2017-08-31 (3.20.0)} {
<li> Added the LSM1 extension
<li> Added the stmt virtual table extension
<li> Added the [sqlite3_prepare_v3()] and [sqlite3_prepare16_v3()] interfaces
     with the extra "prepFlags" parameters.
<li> Provide the [SQLITE_PREPARE_PERSISTENT] flag [sqlite3_prepare_v3()] and
     use it to limit [lookaside memory] misuse by [FTS3], [FTS5], and the
     [R-Tree extension]
<li> Enhancements to the [command-line shell]:
<ul>







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  set aChng($nChng) [list $date $desc $options]
  set xrefChng($date) $nChng
  incr nChng
}

chng {2017-08-31 (3.20.0)} {
<li> Added the LSM1 extension
<li> Added the [STMT virtual table] extension
<li> Added the [sqlite3_prepare_v3()] and [sqlite3_prepare16_v3()] interfaces
     with the extra "prepFlags" parameters.
<li> Provide the [SQLITE_PREPARE_PERSISTENT] flag [sqlite3_prepare_v3()] and
     use it to limit [lookaside memory] misuse by [FTS3], [FTS5], and the
     [R-Tree extension]
<li> Enhancements to the [command-line shell]:
<ul>

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COMPILE_OPTION {SQLITE_ENABLE_STMT_SCANSTATUS} {
  This option enables the [sqlite3_stmt_scanstatus()] interface.  The
  [sqlite3_stmt_scanstatus()] interface is normally omitted from the build
  because it imposes a small performance penalty, even on statements that
  do not use the feature.
}





COMPILE_OPTION {SQLITE_RTREE_INT_ONLY} {
  If this option is used together with [SQLITE_ENABLE_RTREE] then the
  [rtree | R*Tree extension] will only store 32-bit signed integer
  coordinates and all internal computations will be done using integers
  instead of floating point numbers.
}







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COMPILE_OPTION {SQLITE_ENABLE_STMT_SCANSTATUS} {
  This option enables the [sqlite3_stmt_scanstatus()] interface.  The
  [sqlite3_stmt_scanstatus()] interface is normally omitted from the build
  because it imposes a small performance penalty, even on statements that
  do not use the feature.
}

COMPILE_OPTION {SQLITE_ENABLE_STMTVTAB} {
  This compile-time option enables the [STMT virtual table] logic.
}

COMPILE_OPTION {SQLITE_RTREE_INT_ONLY} {
  If this option is used together with [SQLITE_ENABLE_RTREE] then the
  [rtree | R*Tree extension] will only store 32-bit signed integer
  coordinates and all internal computations will be done using integers
  instead of floating point numbers.
}

Changes to pages/optoverview.in.

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  Examples:
}
CODE {
  SELECT MIN(x) FROM table;
  SELECT MAX(x)+1 FROM table;
}

HEADING 1 {Automatic Indexes} autoindex {automatic indexing} {Automatic indexing}


PARAGRAPH {
  ^(When no indices are available to aid the evaluation of a query, SQLite
  might create an automatic index that lasts only for the duration
  of a single SQL statement.)^
  Since the cost of constructing the automatic index is
  O(NlogN) (where N is the number of entries in the table) and the cost of







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  Examples:
}
CODE {
  SELECT MIN(x) FROM table;
  SELECT MAX(x)+1 FROM table;
}

HEADING 1 {Automatic Indexes} autoindex {automatic indexing} \
{Automatic indexing} {automatic indexes}

PARAGRAPH {
  ^(When no indices are available to aid the evaluation of a query, SQLite
  might create an automatic index that lasts only for the duration
  of a single SQL statement.)^
  Since the cost of constructing the automatic index is
  O(NlogN) (where N is the number of entries in the table) and the cost of

Added pages/stmt.in.



































































































































































































































































































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<title>The STMT Virtual Table</title>
<tcl>hd_keywords stmt {STMT virtual table} \
        {the STMT extension}</tcl>
<fancy_format>

<h1>Overview</h1>

<p>
The STMT extension implements an [eponymous-only virtual table] that
provides information about all [prepared statements] associated with
the [database connection].
</p>

<p>
The STMT extension is included in the [amalgamation] though it is disabled
by default.  The enable the STMT extension, use the [SQLITE_ENABLE_STMTVTAB]
compile-time option.  The STMT extension can also be loaded at run-time
by compiling the extension into a shared library or DLL using the source
code at [https://sqlite.org/src/file/ext/misc/stmt.c] and following the
instructions for how to [compile loadable extensions].
</p>

<p>
The STMT extension is enabled in default builts of the [command-line shell].

<h1>Usage</h1>

<p>
The STMT virtual table is a read-only table that can be directly queried
to access information about all prepared statements on the current database
connection.  For example:

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

<p>
A statement such as the above can be run immediately prior to invoking
[sqlite3_close()] to confirm that all prepared statements have been 
[sqlite3_finalize|finalized] and to help identify and track down prepared
statements that have "leaked" and missed finalization.

<p>
The STMT virtual table can also be used to access performance information
about prepared statements, to aid in optimization an application.  For example,
to find out how much memory is being used by [prepared statements] that have
never been used, once could run:

<codeblock>
SELECT sum(mem) FROM stmt WHERE run=0;
</codeblock>

<h2>Columns</h2>

<p>
The columns are provided by the STMT virtual table are summarized by
the hypothetical CREATE TABLE statement show here:

<codeblock>
CREATE TABLE stmt(
  ptr    INT,     -- Numeric value of the pointer to sqlite3_stmt
  sql    TEXT,    -- Original SQL text
  ncol   INT,     -- Number of output columns
  ro     BOOLEAN, -- True for "read only" statements
  busy   BOOLEAN, -- True if the statement is current running
  nscan  INT,     -- Number of full-scan steps
  nsort  INT,     -- Number of sort operations
  naidx  INT,     -- Number of automatic index inserts
  nstep  INT,     -- Number of byte-code engine steps
  reprep INT,     -- Number of reprepare operations
  run    INT,     -- Number of times this statement has been run
  mem    INT      -- Heap memory used by this statement
);
</codeblock>

<p>Future releases may add new output columns and may change the order
of legacy columns.
Further detail about the meaning of each column in the STMT virtual
table is provided below:

<ul>
<li><p><b>ptr</b>:
The numeric value of the pointer to the [sqlite3_stmt] object.

<li><p><b>sql</b>:
The original SQL text of the prepared statement.  If the prepared
statement is compiled using the [sqlite3_prepare()] interface, then
the SQL text might not have been saved, in which case this column
will be NULL.

<li><p><b>ncol</b>:
The number of columns in the result set of a query.
For DML statements, this column has a value of 0.

<li><p><b>ro</b>:
The "read only" column.  This column is true (non-zero) if the
SQL statement is a query and false (zero) if it is a DML statement.

<li><p><b>busy</b>:
This field is true if the prepared statement is currently running.
In other words, this field is true if [sqlite3_step()] has been called
on the [prepared statement] at least once but [sqlite3_reset()] has
not yet been called to reset it.

<li><p><b>nscan</b>:
This field is the number of times that the [bytecode engine] has stepped
through a table as part of a full-table scan.  A large number if this
field may indicate an opportunity to improve performance by adding an
index.  This field is equivalent to the [SQLITE_STMTSTATUS_FULLSCAN_STEP]
value.

<li><p><b>nsort</b>:
This field is the number of times that the [bytecode engine] had to sort.
A positive value in this field may indicate an opportunity to improve
performance by adding an index that will cause the query results to
appear naturally in the desired order.  
This field is equivalent to the [SQLITE_STMTSTATUS_SORT] value.

<li><p><b>naidx</b>:
This field is the number of rows that have been inserted into
[automatic indexes].  A positive value in this field may indicate 
an opportunity to improve performance by adding a named index that
take the place of the automatic index.
This field is equivalent to the [SQLITE_STMTSTATUS_AUTOINDEX] value.

<li><p><b>nstep</b>:
This field is the number of [bytecode engine] operations that have
been performed for the prepared statement.  This field can be used
as a proxy for how much CPU time a statement has used.
This field is equivalent to the [SQLITE_STMTSTATUS_VM_STEP] value.

<li><p><b>reprep</b>:
This field is the number of times that the statement has had to be
reprepared due to schema changes or changes to parameter bindings.
This field is equivalent to the [SQLITE_STMTSTATUS_REPREPARE] value.

<li><p><b>run</b>:
This field is the number of times that the statement has been run.
This field is equivalent to the [SQLITE_STMTSTATUS_RUN] value.

<li><p><b>mem</b>:
This field is the number of bytes of heap storage used by the
prepared statement.
This field is equivalent to the [SQLITE_STMTSTATUS_MEMUSED] value.
</ul>