## 52.49. `pg_statistic`

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 The catalog `pg_statistic` stores statistical data about the contents of the database. Entries are created by [`ANALYZE`](sql-analyze.html) 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`](view-pg-stats.html) is a publicly readable view on `pg_statistic` that only exposes information about those tables that are readable by the current user.

**Table 52.49. `pg_statistic` Columns**

|                                                                                                                                                                                                                              Column Type<br/><br/> Description                                                                                                                                                                                                                              |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
|                                                                                                                                                                          `starelid` `oid` (references [`pg_class`](catalog-pg-class.html).`oid`)<br/><br/> The table or index that the described column belongs to                                                                                                                                                                          |
|                                                                                                                                                                              `staattnum` `int2` (references [`pg_attribute`](catalog-pg-attribute.html).`attnum`)<br/><br/> The number of the described column                                                                                                                                                                              |
|                                                                                                                                                                              `stainherit` `bool`<br/><br/> If true, the stats include inheritance child columns, not just the values in the specified relation                                                                                                                                                                              |
|                                                                                                                                                                                                     `stanullfrac` `float4`<br/><br/> The fraction of the column's entries that are null                                                                                                                                                                                                     |
|                                                                                                                                                                                                     `stawidth` `int4`<br/><br/> The average stored width, in bytes, of nonnull entries                                                                                                                                                                                                      |
|`stadistinct` `float4`<br/><br/> 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`<br/><br/> 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`](catalog-pg-operator.html).`oid`)<br/><br/> 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`](catalog-pg-collation.html).`oid`)<br/><br/> 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[]`<br/><br/> 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`<br/><br/> 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`.                                                    |

