51.51. pg_statistic#
The catalogpg_statistic stores statistical data about the contents of the database. Entries are created byANALYZE 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, withstainherit =false, for each table column that has been analyzed. If the table has inheritance children or partitions, a second entry withstainherit =true is also created. This row represents the column's statistics over the inheritance tree, i.e., statistics for the data you'd see withSELECT, whereas thecolumn FROMtable*stainherit =false row represents the results ofSELECT.column FROM ONLYtable
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 havestainherit =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 inpg_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 seesrc/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 onpg_statistic that only exposes information about those tables that are readable by the current user.
Table 51.51. pg_statistic Columns
Column Type Description |
|---|
The table or index that the described column belongs to |
The number of the described column |
If true, the stats include values from child tables, not just the values in the specified relation |
The fraction of the column's entries that are null |
The average stored width, in bytes, of nonnull entries |
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 |
A code number indicating the kind of statistics stored in the |
An operator used to derive the statistics stored in the |
The collation used to derive the statistics stored in the |
Numerical statistics of the appropriate kind for the |
Column data values of the appropriate kind for the |