pg_statistic

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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 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.

When 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 gpsd and 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.
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 int2 pg_attribute.attnum The number of the described column.
stainherit bool   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 int4   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 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 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, so there is no way to define these columns' type more specifically than anyarray.