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Imposter Tables

1. Introduction

An imposter table is a table that is attached to the same b-tree as an index. An imposter table allows the content of an index to be queried or modified as if the index were an ordinary table.

Imposter tables are intended for analysis and debugging only. This is not a feature that most application developers should understand or even know about. Imposter tables are for experts only.

Improper use of imposter tables can cause index corruption, though any corruption created this way can be fixed by running REINDEX.

2. Details

Each table and each index in SQLite is stored in a separate b-tree in the database file. Each b-tree is identified by its root page number. The root page number for any index or table can be found by querying the "rootpage" column of the sqlite_master table. See the indexing tutorial and the file format documentation for further background on this design.

Usually the b-trees for tables and indexes are slightly different. A table b-tree contains a 64-bit integer key and arbitrary data. The 64-bit integer key is the ROWID. Index b-trees contain an arbitrary binary key and no data. So table b-trees and index b-trees are not directly compatible.

However, the b-tree for a WITHOUT ROWID table is in the same format as an index b-tree. Thus, an index b-tree can be accessed as if it were a WITHOUT ROWID table.

2.1. Manually Created Imposter Tables

One way to create an imposter table is to directly edit the sqlite_master table to insert a new row that describes the table. For example, suppose the schema is like this:

CREATE TABLE t1(a INTEGER PRIMARY KEY,b TEXT,c INT, d INT);
CREATE INDEX t1bc ON t1(b,c);

The WITHOUT ROWID table that has the same structure as the t1bc index would look like this:

CREATE TABLE t2(b TEXT,c INT,a INT, PRIMARY KEY(b,c,a)) WITHOUT ROWID;

To create a permanent imposter table "t2" against index "t1bc" one should first enable editing of the sqlite_master table by running "PRAGMA writable_schema=ON". (Be careful to observe the warnings that accompany this PRAGMA. A mistake can cause severe database corruption.) Then insert a new entry into the sqlite_master table like this:

INSERT INTO sqlite_master(type,name,tbl_name,rootpage,sql)
 SELECT 'table','t2','t2',rootpage,
   'CREATE TABLE t2(b,c,a,PRIMARY KEY(b,c,a))WITHOUT ROWID'
   FROM sqlite_master
  WHERE name='t1bc';

The INSERT statement above adds a new row to the sqlite_master table that defines a table "t2" having the same on-disk format as index "t1bc" and pointing to the same b-tree. After adding this sqlite_master table entry, it is necessary to close and reopen the database in order to get SQLite to reread the schema. Then the "t2" table can be queried to see the content of the "t1bc" index.

2.1.1. Corrupted Database

A serious problem with the manual imposter table approach described above is that after adding the new "t2" entry to the "sqlite_master" table, the database file will technically be corrupt. Both the "t1bc" index and the "t2" table will point to the same b-tree. This will not cause any immediate problems, though one should avoid running VACUUM.

It is possible to write into the "t2" table, thus changing the content of the index. But doing so will get the "t1bc" index out of synchronization with its parent table "t1". An out-of-sync index can result in incorrect query results.

Since the "t2" imposter table is a form of database corruption, the manual approach to creating imposter tables is not recommended. Actually, any use of imposter tables is discouraged for all but expert developers, but manually created imposter tables are especially discouraged because they are permanent.

2.2. Transient Imposter Tables

Another (safer) approach to creating an imposter table is to add an entry for the imposter table to SQLite's internal symbol table without updating the "sqlite_master" table on disk. That way, the imposter table exists in only a single database connection and is automatically removed whenever the schema is reloaded.

Creation of a transient imposter table involves a special sqlite3_test_control() call. Unlike all other SQLite APIs, sqlite3_test_control() interface is subject to incompatible changes from one release to the next, and so the mechanism described below is not guaranteed to work in future releases of SQLite. The SQLite developers do not consider this a problem because imposter tables should not be used in applications. Imposter tables are for analysis and testing use only.

To create a transient imposter table, first call sqlite3_test_control() as follows:

sqlite3_test_control(SQLITE_TESTCTRL_IMPOSTER, db, "main", 1, tnum);

The "db" parameter is a pointer to the database connection. The "main" argument is the name of the schema in which the imposter table is to be created. The "1" argument enables the imposter table mechanism. "tnum" is the root page of the index that the imposter table should mirror.

After the sqlite3_test_control() call above, then run a CREATE TABLE statement the defines the imposter table. With the imposter mechanism enabled, this CREATE TABLE statement does not create a real table but instead merely adds an entry in SQLite's internal symbol table. Note that the CREATE TABLE statement must be in the correct format for the index. If the imposter table has the wrong number of columns or is not a WITHOUT ROWID table or is otherwise incompatible with the index b-tree, then SQLITE_CORRUPT errors will result when the imposter table is used.

After running the CREATE TABLE statement, disable the imposter mechanism as follows:

sqlite3_test_control(SQLITE_TESTCTRL_IMPOSTER, db, "main", 0, 0);

In other words, make the same sqlite3_test_control() call except change the last two parameters to zero.

After the imposter table is loaded into SQLite's internal schema as described above, the imposter table can be used as any other table. But the imposter table will only be visible to the one database connection that created it. No changes are made to the database file on disk. And the imposter table will disappear the next time the schema is loaded.

2.3. The .imposter Shell Command

As of SQLite 3.16.0 (2017-01-02), the command-line shell contains a dot-command ".imposter" that does all of the work of setting up a transient imposter table. Instead of making multiple calls to sqlite3_test_control() and figuring out and invoking a compatible CREATE TABLE statement, a transient imposter table can be constructed as follows:

.imposter t1bc t2

Of course, substitute the desired index and imposter table names in place of the "t1bc" and "t2" shown in the example. The ".imposter" command reads the schema of the "t1bc" index, uses that information to construct a compatible CREATE TABLE statement for the imposter table, then makes all the necessary calls to create the transient imposter table automatically.

3. Summary And Final Warning

The imposter table mechanism is a power analysis and debugging tool for SQLite. But as with all sharp tools, it can also be dangerous and can result in corrupt database files if misused. Do not attempt to use imposter tables in an application. Imposter tables are intended for use in the laboratory by experts.

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