

PostgreSQL 8.1.4 Documentation  

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The catalog 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 uptodate.
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 nonexpression index column, however, since it would be redundant with the entry for the underlying table column.
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. 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.
Table 4230. pg_statistic Columns
Name  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 
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. 