The SQLITE_DBPAGE Virtual Table

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The SQLITE_DBPAGE Virtual Table

# 1. Overview

The SQLITE_DBPAGE extension implements an eponymous-only virtual table that provides direct access to the underlying database file by interacting with the pager. SQLITE_DBPAGE is capable of both reading and writing any page of the database. Because interaction is through the pager layer, all changes are transactional.

Warning: writing to the SQLITE_DBPAGE virtual table can very easily cause unrecoverable database corruption. Do not allow untrusted components to access the SQLITE_DBPAGE table. Use appropriate care while using the SQLITE_DBPAGE table. Back up important data prior to experimenting with the SQLITE_DBPAGE table. Writes to the SQLITE_DBPAGE virtual table are disabled when the SQLITE_DBCONFIG_DEFENSIVE flag is set.

The SQLITE_DBPAGE extension is included in the amalgamation though it is disabled by default. Use the SQLITE_ENABLE_DBPAGE_VTAB compile-time option to enable the SQLITE_DBPAGE extension. The SQLITE_DBPAGE extension makes use of unpublished internal interfaces and is not run-time loadable. The only way to add SQLITE_DBPAGE to an application is to compile it in using the SQLITE_ENABLE_DBPAGE_VTAB compile-time option.

The SQLITE_DBPAGE extension is enabled in default builds of the command-line shell, but is read-only by default. Use the --unsafe-testing command-line option to the CLI to enable writing to the sqlite_dbpage virtual table.

# 2. Usage

The SQLITE_DBPAGE virtual table read/write table that provides direct access to the underlying disk file on a page-by-page basis. The virtual table appears to have a schema like this:

    CREATE TABLE sqlite_dbpage(
      pgno INTEGER PRIMARY KEY,
      data BLOB
    );

An SQLite database file is divided into pages. The first page is 1, the second page is 2, and so forth. There is no page 0. Every page is the same size. The size of every page is a power of 2 between 512 and 65536. See the file format documentation for further details.

The SQLITE_DBPAGE table allows an application to view or replace the raw binary content of each page of the database file. No attempt is made to interpret the content of the page. Content is returned byte-for-byte as it appears on disk.

The SQLITE_DBPAGE table has one row for each page in the database file. SQLITE_DBPAGE allows pages to be read using a SELECT statement or to be overwritten using an UPDATE or INSERT statement. When using an INSERT against sqlite_dbpage, it acts like REPLACE. In other words, INSERT into a page that already exists overwrites that page. To increase the size of a database, INSERT into a page beyond the end of the database.

## 2.1. Using SQLITE_DBPAGE On ATTACH-ed Databases

The SQLITE_DBPAGE table schema shown above is incomplete. There is a third hidden column named "schema" that determines which ATTACH-ed database should be read or written. Because the "schema" column is hidden, it can be used as a parameter when SQLITE_DBPAGE is invoked as a table-valued function.

For example, suppose an additional database is attached to the database connection using a statement like this:

    ATTACH 'auxdata1.db' AS aux1;

Then to read the first page of that database file, one merely runs:

    SELECT data FROM sqlite_dbpage('aux1') WHERE pgno=1;

If the "schema" is omitted, it defaults to the primary database (usually called 'main', unless renamed using SQLITE_DBCONFIG_MAINDBNAME). Hence, the following two queries are normally equivalent:

    SELECT data FROM sqlite_dbpage('main') WHERE pgno=1;
    SELECT data FROM sqlite_dbpage WHERE pgno=1;

The SQLITE_DBPAGE table can participate in a join just like any other table. Hence, to see the content of the first page to all connected database files, one might run a statement like this:

    SELECT dbpage.data, dblist.name
      FROM pragma_database_list AS dblist
      JOIN sqlite_dbpage(dblist.name) AS dbpage
     WHERE dbpage.pgno=1;

## 2.2. Truncating A Database

Normally the data for UPDATE and INSERT must be a blob which is the exact same size as a database page. However, as a special case, doing an INSERT of a NULL into page 2 or greater as the last operation prior to COMMIT causes the page being inserted into and all subsequent pages to be deleted, truncating the database. For example, a command like "`INSERT INTO sqlite_dbpage(pgno,data) VALUES(52,NULL)`" causes the database to be truncated to 51 pages in length. Pages are numbered beginning with 1, so the total number of pages remaining is the same as the last page number or 51 in this case.

Call an INSERT into sqlite_dbpage with a NULL page content a "null-INSERT". The null-INSERT behavior is a special case and does not necessarily follow the usual rules. The following are some specific limitations of null-INSERT:

1. The sqlite_dbpage table does not allow a null-INSERT into page 1. Hence, it is not possible to truncate a database to zero bytes or zero pages using sqlite_dbpage, as doing so would omit the database header and the file would cease to be an SQLite database.

2. A null-INSERT only truncates the database if it is the last write operation to occur before the transaction commit. If the null-INSERT is within a multi-statement transaction and is followed by one or more UPDATE or INSERT operations within that same transaction, then the null-INSERT becomes a no-op and the database is not truncated.

3. Only a single database file can be truncated per transaction. The sqlite_dbpage virtual table is able to write to multiple ATTACH-ed databases during the same transaction, but because a null-INSERT must be the last write statement to occur before COMMIT, sqlite_dbpage can only truncate one of the ATTACH-ed databases. If multiple ATTACH-ed databases need to be truncated, each must be truncated in a separate transaction.

4. Any ROLLBACK TO will cancel the null-INSERT, even a ROLLBACK TO that does not involve the null-INSERT itself. The special behavior of null-INSERT is only guaranteed if it occurs as its own transaction or immediately before COMMIT.

# 3. Compatibility

The ability to do an INSERT into sqlite_dbpage, and to increase or decrease the size of the database using INSERT was added in SQLite version 3.47.0 (2024-10-21). Older versions do not allow INSERT against sqlite_dbpage.

This page last modified on 2024-10-22 19:45:21 UTC