The DBSTAT Virtual Table

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The DBSTAT Virtual Table

# 1. Overview

The DBSTAT virtual table is a read-only eponymous virtual table that returns information about the amount of disk space used to store the content of an SQLite database. Example use cases for the DBSTAT virtual table include the sqlite3_analyzer.exe utility program and the table size pie-chart in the Fossil-implemented version control system for SQLite.

The DBSTAT virtual table is available on all database connections when SQLite is built using the SQLITE_ENABLE_DBSTAT_VTAB compile-time option.

The DBSTAT virtual table is an eponymous virtual table, meaning that is not necessary to run CREATE VIRTUAL TABLE to create an instance of the dbstat virtual table before using it. The "dbstat" module name can be used as if it were a table name to query the dbstat virtual table directly. For example:

    SELECT * FROM dbstat;

If a named virtual table that uses the dbstat module is desired, then the recommended way to create an instance of the dbstat virtual table is as follows:

    CREATE VIRTUAL TABLE temp.stat USING dbstat(main);

Note the "temp." qualifier before the virtual table name ("stat"). This qualifier causes the virtual table to be temporary - to only exist for the duration of the current database connection. This is the recommended approach.

The "main" argument to dbstat is default schema for which information is to be provided. The default is "main", and so the use of "main" in the example above is redundant. For any particular query, the schema can be changed by specifying the alternative schema as a function argument to the virtual table name in the FROM clause of the query. (See further discussion of table-valued functions in the FROM clause for more details.)

The schema for the DBSTAT virtual table looks like this:

    CREATE TABLE dbstat(
      name       TEXT,        -- Name of table or index
      path       TEXT,        -- Path to page from root
      pageno     INTEGER,     -- Page number, or page count
      pagetype   TEXT,        -- 'internal', 'leaf', 'overflow', or NULL
      ncell      INTEGER,     -- Cells on page (0 for overflow pages)
      payload    INTEGER,     -- Bytes of payload on this page or btree
      unused     INTEGER,     -- Bytes of unused space on this page or btree
      mx_payload INTEGER,     -- Largest payload size of all cells on this row
      pgoffset   INTEGER,     -- Byte offset of the page in the database file
      pgsize     INTEGER,     -- Size of the page, in bytes
      schema     TEXT HIDDEN, -- Database schema being analyzed
      aggregate  BOOL HIDDEN  -- True to enable aggregate mode
    );

The DBSTAT table only reports on the content of btrees within the database file. Freelist pages, pointer-map pages, and the lock page are omitted from the analysis.

By default, there is a single row in the DBSTAT table for each btree page the database file. Each row provides information about the space utilization of that one page of the database. However, if the hidden column "aggregate" is TRUE, then results are aggregated and there is a single row in the DBSTAT table for each btree in the database, providing information about space utilization across the entire btree.

# 2. The "path" column of the dbstat virtual table

The "path" column describes the path taken from the root node of the btree structure to each page. The "path" of the root node itself is '/'. The "path" is NULL when "aggregate" is TRUE. The "path" for the left-most child page of the root of a btree page is '/000/'. (Btrees store content ordered from left to right so the pages to the left have smaller keys than the pages to the right.) The next to left-most child of the root page is '/001', and so on, each sibling page identified by a 3-digit hex value. The children of the 451st left-most sibling have paths such as '/1c2/000/, '/1c2/001/' etc. Overflow pages are specified by appending a '+' character and a six-digit hexadecimal value to the path to the cell they are linked from. For example, the three overflow pages in a chain linked from the left-most cell of the 450th child of the root page are identified by the paths:

    '/1c2/000+000000'         // First page in overflow chain
    '/1c2/000+000001'         // Second page in overflow chain
    '/1c2/000+000002'         // Third page in overflow chain

If the paths are sorted using the BINARY collation sequence, then the overflow pages associated with a cell will appear earlier in the sort-order than its child page:

    '/1c2/000/'               // Left-most child of 451st child of root

# 3. Aggregated Data

Beginning with SQLite version 3.31.0 (2020-01-22), the DBSTAT table has a new hidden column named "aggregate", which if constrained to be TRUE will cause DBSTAT to generate one row per btree in the database, rather than one row per page. When running in aggregated mode, the "path", "pagetype", and "pgoffset" columns are always NULL and the "pageno" column holds the number of pages in the entire btree, rather than the number of the page that corresponds to the row.

The following table shows the meanings of the (non-hidden) columns of DBSTAT in both normal and aggregated mode:

> Column | Normal meaning | Aggregate-mode meaning
> --- | --- | ---
> name | The name of the table or index that is implemented by the btree of the current row
> path | See description above | Always NULL
> pageno | The page number of the database page for the current row | The total number of pages in the btree for the current row
> pagetype | 'leaf' or 'interior' | Always NULL
> ncell | Number of cells on the current page or btree
> payload | Bytes of useful payload on the current page or btree
> unused | Unused bytes of on the current page or btree
> mx_payload | The largest payload found anywhere in the current page or btree.
> pgoffset | Byte offset to the start of the page | Always NULL
> pgsize | Total storage space used by the current page or btree.

# 4. Example uses of the dbstat virtual table

To find the total number of pages used to store table "xyz" in schema "aux1", use either of the following two queries (the first is the traditional way, and the second shows the use of the aggregated feature):

    SELECT count(*) FROM dbstat('aux1') WHERE name='xyz';
    SELECT pageno FROM dbstat('aux1',1) WHERE name='xyz';

To see how efficiently the content of a table is stored on disk, compute the amount of space used to hold actual content divided by the total amount of disk space used. The closer this number is to 100%, the more efficient the packing. (In this example, the 'xyz' table is assumed to be in the 'main' schema. Again, there are two different versions that show the use of DBSTAT both without and with the new aggregated feature, respectively.)

    SELECT sum(pgsize-unused)*100.0/sum(pgsize) FROM dbstat WHERE name='xyz';
    SELECT (pgsize-unused)*100.0/pgsize FROM dbstat
     WHERE name='xyz' AND aggregate=TRUE;

To find the average fan-out for a table, run:

    SELECT avg(ncell) FROM dbstat WHERE name='xyz' AND pagetype='internal';

Modern filesystems operate faster when disk accesses are sequential. Hence, SQLite will run faster if the content of the database file is on sequential pages. To find out what fraction of the pages in a database are sequential (and thus obtain a measurement that might be useful in determining when to VACUUM), run a query like the following:

    CREATE TEMP TABLE s(rowid INTEGER PRIMARY KEY, pageno INT);
    INSERT INTO s(pageno) SELECT pageno FROM dbstat ORDER BY path;
    SELECT sum(s1.pageno+1==s2.pageno)*1.0/count(*)
      FROM s AS s1, s AS s2
     WHERE s1.rowid+1=s2.rowid;
    DROP TABLE s;

This page last modified on 2020-01-22 17:49:40 UTC