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 optimizer. There is one entry for each table column that has been analyzed. Note that all the statistical data is inherently approximate, even assuming that it is up-to-date.
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.
stainherit = false, there is normally one entry 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.
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.
Statistical information about a table's contents should be considered sensitive (for 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.
Warning: Diagnostic tools such as
minirepro collect sensitive information from
pg_statistic, such as histogram boundaries, in a clear, readable form. Always review the output files of these utilities to ensure that the contents are acceptable for transport outside of the database in your organization.
||oid||pg_class.oid||The table or index that the described column belongs to.|
||int2||pg_attribute.attnum||The number of the described column.|
||bool||If true, the statistics include inheritance child columns, not just the values in the specified relations.|
||float4||The fraction of the column's entries that are null.|
||int4||The average stored width, in bytes, of nonnull entries.|
||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 fraction of the number of rows in the table (for example, a column in which values appear about twice on the average could be represented by
||int2||A code number indicating the kind of statistics stored in the
||oid||pg_operator.oid||An operator used to derive the statistics stored in the
||float4||Numerical statistics of the appropriate kind for the
||anyarray||Column data values of the appropriate kind for the
Parent topic: System Catalogs Definitions