pg_statistic

PostgreSQL 9.0.4 Documentation
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# 45.36. `pg_statistic`

The catalog `pg_statistic` stores statistical data about the contents of the database. Entries are created by ANALYZE and subsequently used by the query planner. Note that all the statistical data is inherently approximate, even assuming that it is up-to-date.

Normally there is one entry, with `stainherit` = `false`, for each table column that has been analyzed. If the table has inheritance children, a second entry with `stainherit` = `true` is also created. This row represents the column's statistics over the inheritance tree, i.e., statistics for the data you'd see with `SELECT `column` FROM `table`*`, whereas the `stainherit` = `false` row represents the results of `SELECT `column` FROM ONLY `table``.

`pg_statistic` also stores statistical data about the values of index expressions. These are described as if they were actual data columns; in particular, `starelid` references the index. No entry is made for an ordinary non-expression index column, however, since it would be redundant with the entry for the underlying table column. Currently, entries for index expressions always have `stainherit` = `false`.

Since different kinds of statistics might be appropriate for different kinds of data, `pg_statistic` is designed not to assume very much about what sort of statistics it stores. Only extremely general statistics (such as nullness) are given dedicated columns in `pg_statistic`. Everything else is stored in "slots", which are groups of associated columns whose content is identified by a code number in one of the slot's columns. For more information see `src/include/catalog/pg_statistic.h`.

`pg_statistic` should not be readable by the public, since even statistical information about a table's contents might be considered sensitive. (Example: minimum and maximum values of a salary column might be quite interesting.) `pg_stats` is a publicly readable view on `pg_statistic` that only exposes information about those tables that are readable by the current user.

Table 45-36. `pg_statistic` Columns

Name | Type | References | Description
--- | --- | --- | ---
`starelid` | `oid` | ``pg_class`.oid` | The table or index that the described column belongs to
`staattnum` | `int2` | ``pg_attribute`.attnum` | The number of the described column
`stainherit` | `bool` |  | If true, the stats include inheritance child columns, not just the values in the specified relation
`stanullfrac` | `float4` |  | The fraction of the column's entries that are null
`stawidth` | `int4` |  | The average stored width, in bytes, of nonnull entries
`stadistinct` | `float4` |  | The number of distinct nonnull data values in the column. A value greater than zero is the actual number of distinct values. A value less than zero is the negative of a multiplier for the number of rows in the table; for example, a column in which values appear about twice on the average could be represented by `stadistinct` = -0.5. A zero value means the number of distinct values is unknown.
`stakind`N`` | `int2` |  | A code number indicating the kind of statistics stored in the `N`th "slot" of the `pg_statistic` row.
`staop`N`` | `oid` | ``pg_operator`.oid` | An operator used to derive the statistics stored in the `N`th "slot". For example, a histogram slot would show the `<` operator that defines the sort order of the data.
`stanumbers`N`` | `float4[]` |  | Numerical statistics of the appropriate kind for the `N`th "slot", or null if the slot kind does not involve numerical values
`stavalues`N`` | `anyarray` |  | Column data values of the appropriate kind for the `N`th "slot", or null if the slot kind does not store any data values. Each array's element values are actually of the specific column's data type, so there is no way to define these columns' type more specifically than `anyarray`.

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