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The SQLITE_MEMSTAT Virtual Table

# 1. Overview

The SQLITE_MEMSTAT extension implements an eponymous-only virtual table that provides SQL access to the sqlite3_status64() and sqlite3_db_status() interfaces.

The SQLITE_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/memstat.c and following the instructions for how to compile loadable extensions.

# 2. Usage

The SQLITE_MEMSTAT virtual table is a read-only table that can be queried to determine performance characteristics (primarily the amount of memory being used) of the current instance of SQLite. The SQLITE_MEMSTATE table is essentially a wrapper around the C-language APIs sqlite3_status64() and sqlite3_db_status(). If the memstat.c source file is compiled with the -DSQLITE_ENABLE_ZIPVFS option, then SQLITE_MEMSTAT will also do some file-control calls to extract memory usage information about the ZIPVFS subsystem, if that subsystem as been licensed, installed, and is in use.

The SQLITE_MEMSTAT table appears to have the following schema:

    CREATE TABLE sqlite_memstat(
      name TEXT,
      schema TEXT,
      value INT,
      hiwtr INT
    );

Each row of the SQLITE_MEMSTAT table corresponds to a single call to one of the sqlite3_status64() or sqlite3_db_status() interfaces. The NAME column of the row identifies which "verb" was passed to those interfaces. For example, if sqlite3_status64() is invoked with SQLITE_STATUS_MEMORY_USED, then the NAME column is 'MEMORY_USED'. Or if sqlite3_db_status() is invoked with SQLITE_DBSTATUS_CACHE_USED, then the NAME column is "DB_CACHE_USED".

The SCHEMA column is NULL, except for cases when the sqlite3_file_control() interface is used to interrogate the ZIPVFS backend. As this only happens when the memstat.c module is compiled with -DSQLITE_ENABLE_ZIPVFS and when ZIPVFS is in use, SCHEMA is usually NULL.

The VALUE and HIWTR columns report the current value of the measure and its "high-water mark". The high-water mark is the highest value ever seen for the measurement, at least since the last reset. The SQLITE_MEMSTAT virtual table does not provide a mechanism for resetting the high-water mark.

Depending on which parameter is being interrogated, one of the VALUE or HIWTR mark measurements might be undefined. For example, only the high-water mark is meaningful for SQLITE_STATUS_MALLOC_SIZE, and only the current value is meaningful for SQLITE_DBSTATUS_CACHE_USED. For rows where one or the other of VALUE or HIWTR is not meaningful, that value is returned as NULL. the interfaces, with the initial

This page last modified on 2018-09-28 23:43:25 UTC