53.31. pg_stats_ext_exprs
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## 53.31. pg_stats_ext_exprs #

The view pg_stats_ext_exprs provides access to information about all expressions included in extended statistics objects, combining information stored in the pg_statistic_ext and pg_statistic_ext_data catalogs. This view allows access only to rows of pg_statistic_ext and pg_statistic_ext_data that correspond to tables the user owns, and therefore it is safe to allow public read access to this view.

pg_stats_ext_exprs is also designed to present the information in a more readable format than the underlying catalogs — at the cost that its schema must be extended whenever the structure of statistics in pg_statistic_ext changes.

Table 53.31. pg_stats_ext_exprs Columns

Column Type Description
`schemaname` name (references pg_namespace.`nspname`) Name of schema containing table
`tablename` name (references pg_class.`relname`) Name of table the statistics object is defined on
`statistics_schemaname` name (references pg_namespace.`nspname`) Name of schema containing extended statistics object
`statistics_name` name (references pg_statistic_ext.`stxname`) Name of extended statistics object
`statistics_owner` name (references pg_authid.`rolname`) Owner of the extended statistics object
`expr` text Expression included in the extended statistics object
`inherited` bool (references pg_statistic_ext_data.`stxdinherit`) If true, the stats include values from child tables, not just the values in the specified relation
`null_frac` float4 Fraction of expression entries that are null
`avg_width` int4 Average width in bytes of expression's entries
`n_distinct` float4 If greater than zero, the estimated number of distinct values in the expression. 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 expression seems to have a fixed number of possible values.) For example, -1 indicates a unique expression 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 expression. (Null if no values seem to be more common than any others.)
`most_common_freqs` float4[] A list of the frequencies of the most common values, i.e., number of occurrences of each divided by total number of rows. (Null when `most_common_vals` is.)
`histogram_bounds` anyarray A list of values that divide the expression's values into groups of approximately equal population. The values in `most_common_vals`, if present, are omitted from this histogram calculation. (This expression is null if the expression data type does not have a `<` operator or if the `most_common_vals` list accounts for the entire population.)
`correlation` float4 Statistical correlation between physical row ordering and logical ordering of the expression values. This ranges from -1 to +1. When the value is near -1 or +1, an index scan on the expression will be estimated to be cheaper than when it is near zero, due to reduction of random access to the disk. (This expression is null if the expression's data type does not have a `<` operator.)
`most_common_elems` anyarray A list of non-null element values most often appearing within values of the expression. (Null for scalar types.)
`most_common_elem_freqs` float4[] A list of the frequencies of the most common element values, i.e., the fraction of rows containing at least one instance of the given value. Two or three additional values follow the per-element frequencies; these are the minimum and maximum of the preceding per-element frequencies, and optionally the frequency of null elements. (Null when `most_common_elems` is.)
`elem_count_histogram` float4[] A histogram of the counts of distinct non-null element values within the values of the expression, followed by the average number of distinct non-null elements. (Null for scalar types.)

The maximum number of entries in the array fields can be controlled on a column-by-column basis using the ALTER TABLE SET STATISTICS command, or globally by setting the default_statistics_target run-time parameter.

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