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44.55. pg_stats

The view pg_stats provides access to the information stored in the pg_statistic catalog. This view allows access only to rows of pg_statistic that correspond to tables the user has permission to read, and therefore it is safe to allow public read access to this view.

pg_stats is also designed to present the information in a more readable format than the underlying catalog — at the cost that its schema must be extended whenever new slot types are defined for pg_statistic.

Table 44-56. pg_stats Columns

Name Type References Description
schemaname name pg_namespace.nspname Name of schema containing table
tablename name pg_class.relname Name of table
attname name pg_attribute.attname Name of the column described by this row
null_frac real   Fraction of column entries that are null
avg_width integer   Average width in bytes of column's entries
n_distinct real   If greater than zero, the estimated number of distinct values in the column. If less than zero, the negative of the number of distinct values divided by the number of rows. (The negated form is used when ANALYZE believes that the number of distinct values is likely to increase as the table grows; the positive form is used when the column seems to have a fixed number of possible values.) For example, -1 indicates a unique column in which the number of distinct values is the same as the number of rows
most_common_vals anyarray   A list of the most common values in the column. (NULL if no values seem to be more common than any others.) For some datatypes such as tsvector, this is a list of the most common element values rather than values of the type itself.
most_common_freqs real[]   A list of the frequencies of the most common values or elements, i.e., number of occurrences of each divided by total number of rows. (NULL when most_common_vals is.) For some datatypes such as tsvector, it can also store some additional information, making it longer than the most_common_vals array.
histogram_bounds anyarray   A list of values that divide the column's values into groups of approximately equal population. The values in most_common_vals, if present, are omitted from this histogram calculation. (This column is NULL if the column data type does not have a < operator or if the most_common_vals list accounts for the entire population.)
correlation real   Statistical correlation between physical row ordering and logical ordering of the column values. This ranges from -1 to +1. When the value is near -1 or +1, an index scan on the column will be estimated to be cheaper than when it is near zero, due to reduction of random access to the disk. (This column is NULL if the column data type does not have a < operator.)

The maximum number of entries in the most_common_vals and histogram_bounds arrays can be set on a column-by-column basis using the ALTER TABLE SET STATISTICS command, or globally by setting the default_statistics_target run-time parameter.