51.51. `pg_statistic`
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## 51.51. `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 or partitions, 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 51.51. `pg_statistic` Columns

Column Type Description
`starelid` `oid` (references `pg_class`.`oid`) The table or index that the described column belongs to
`staattnum` `int2` (references `pg_attribute`.`attnum`) The number of the described column
`stainherit` `bool` If true, the stats include values from child tables, 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 about 80% of the values are nonnull and each nonnull value appears about twice on average could be represented by `stadistinct` = -0.4. 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` (references `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. Zero if the statistics kind does not require an operator.
`stacoll`N`` `oid` (references `pg_collation`.`oid`) The collation used to derive the statistics stored in the `N`th “slot”. For example, a histogram slot for a collatable column would show the collation that defines the sort order of the data. Zero for noncollatable 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, or a related type such as an array's element type, so there is no way to define these columns' type more specifically than `anyarray`.

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