pg_statistic stores statistical data about the contents of the database. Entries are created by ANALYZE and subsequently used by the query planner. 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.
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 NULL-ness) 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 may 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. pg_stats is also designed to present the information in a more readable format than the underlying pg_statistic table --- at the cost that its schema must be extended whenever new slot types are added.
Table 3-20. pg_statistic Columns
|starelid||oid||pg_class.oid||The table that the described column belongs to|
|staattnum||int2||pg_attribute.attnum||The number of the described column|
|stanullfrac||float4||The fraction of the column's entries that are NULL|
|stawidth||int4||The average stored width, in bytes, of non-NULL entries|
|stadistinct||float4||The number of distinct non-NULL 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 stadistinct = -0.5). A zero value means the number of distinct values is unknown.|
|stakindN||int2||A code number indicating the kind of statistics stored in the Nth "slot" of the pg_statistic row.|
|staopN||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.|
|stanumbersN||float4||Numerical statistics of the appropriate kind for the Nth "slot", or NULL if the slot kind does not involve numerical values.|
|stavaluesN||text||Column data values of the appropriate kind for the Nth "slot", or NULL if the slot kind does not store any data values. For data-type independence, all column data values are converted to external textual form and stored as TEXT datums.|