pg_statistic

The pg_statistic system catalog table 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, Greenplum Database creates a second entry with stainherit = true. This row represents the column’s statistics over the inheritance tree, for example, statistics for the data you would 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 may 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.

pg_statistic should not be readable by the public, since even statistical information about a table’s contents should be considered sensitive. (Example: minimum and maximum values of a salary column). 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 1. pg_catalog.pg_statistic

column	type	references	description
starelid	oid	pg_class.oid	The table or index that the described column belongs to
staattnum	smallint	pg_attribute.attnum	The number of the described column
stainherit	boolean		If true, the statistics include inheritance child columns, not just the values in the specified relations.
stanullfrac	float4		The fraction of the column’s entries that are null
stawidth	integer		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 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 the average could be represented by stadistinct = -0.4). A zero value means the number of distinct values is unknown.
stakind*N*	integer		A code number indicating the kind of statistics stored in the Nth slot of the pg_statistic row
staop*N*	oid	pg_operator.oid	An operator used to derive the statistics stored in the Nth slot. For example, a histogram slot would show the < operator that defines the sort order of the data.
kstacoll*N*	oid	pg_collation.oid	The collation used to derive the statistics stored in the Nth 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 Nth slot, or NULL if the slot kind does not involve numerical values.
stavalues*N*	anyarray		Column data values of the appropriate kind for the Nth 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.