<literal>e</literal> for expression statistics

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>stxexprs</structfield> <type>pg_node_tree</type>

</para>

<para>

Expression trees (in <function>nodeToString()</function>

representation) for statistics object attributes that are not simple

column references. This is a list with one element per expression.

Null if all statistics object attributes are simple references.

</para></entry>

</row>

Extended statistics object containing the definition for this data

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>stxexprs</structfield> <type>pg_node_tree</type>

</para>

<para>

A list of any expressions covered by this statistics object.

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>stxdexpr</structfield> <type>pg_statistic[]</type>

</para>

<para>

Per-expression statistics, serialized as an array of

<structname>pg_statistic</structname> type

</para></entry>

</row>

<row>

<entry><link linkend="view-pg-stats-ext-exprs"><structname>pg_stats_ext_exprs</structname></link></entry>

<entry>extended planner statistics for expressions</entry>

</row>

Names of the columns included in the extended statistics object

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>exprs</structfield> <type>text[]</type>

</para>

<para>

Expressions included in the extended statistics object

</para></entry>

</row>

information about each extended statistics object in the database,

combining information stored in the <link

Name of schema containing extended statistics object

Name of extended statistics object

Owner of the extended statistics object

Names of the columns the extended statistics object is defined on

Types of extended statistics object enabled for this record

<sect1 id="view-pg-stats-ext-exprs">

<title><structname>pg_stats_ext_exprs</structname></title>

<indexterm zone="view-pg-stats-ext-exprs">

<primary>pg_stats_ext_exprs</primary>

</indexterm>

<para>

The view <structname>pg_stats_ext_exprs</structname> provides access to

information about all expressions included in extended statistics objects,

combining information stored in the <link

linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link>

and <link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link>

catalogs. This view allows access only to rows of

<link linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link> and <link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link>

that correspond to tables the user has permission to read, and therefore

it is safe to allow public read access to this view.

</para>

<para>

<structname>pg_stats_ext_exprs</structname> is also designed to present

the information in a more readable format than the underlying catalogs

&mdash; at the cost that its schema must be extended whenever the structure

of statistics in <link linkend="catalog-pg-statistic"><structname>pg_statistic</structname></link> changes.

</para>

<table>

<title><structname>pg_stats_ext_exprs</structname> Columns</title>

<tgroup cols="1">

<thead>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

Column Type

</para>

<para>

Description

</para></entry>

</row>

</thead>

<tbody>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>schemaname</structfield> <type>name</type>

(references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)

</para>

<para>

Name of schema containing table

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>tablename</structfield> <type>name</type>

(references <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relname</structfield>)

</para>

<para>

Name of table the statistics object is defined on

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>statistics_schemaname</structfield> <type>name</type>

(references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>nspname</structfield>)

</para>

<para>

Name of schema containing extended statistics object

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>statistics_name</structfield> <type>name</type>

(references <link linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link>.<structfield>stxname</structfield>)

</para>

<para>

Name of extended statistics object

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>statistics_owner</structfield> <type>name</type>

(references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>rolname</structfield>)

</para>

<para>

Owner of the extended statistics object

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>expr</structfield> <type>text</type>

</para>

<para>

Expression included in the extended statistics object

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>null_frac</structfield> <type>float4</type>

</para>

<para>

Fraction of expression entries that are null

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>avg_width</structfield> <type>int4</type>

</para>

<para>

Average width in bytes of expression's entries

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>n_distinct</structfield> <type>float4</type>

</para>

<para>

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

<command>ANALYZE</command> 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.

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>most_common_vals</structfield> <type>anyarray</type>

</para>

<para>

A list of the most common values in the expression. (Null if

no values seem to be more common than any others.)

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>most_common_freqs</structfield> <type>float4[]</type>

</para>

<para>

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 <structfield>most_common_vals</structfield> is.)

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>histogram_bounds</structfield> <type>anyarray</type>

</para>

<para>

A list of values that divide the expression's values into groups of

approximately equal population. The values in

<structfield>most_common_vals</structfield>, if present, are omitted from this

histogram calculation. (This expression is null if the expression data type

does not have a <literal>&lt;</literal> operator or if the

<structfield>most_common_vals</structfield> list accounts for the entire

population.)

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>correlation</structfield> <type>float4</type>

</para>

<para>

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 <literal>&lt;</literal> operator.)

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>most_common_elems</structfield> <type>anyarray</type>

</para>

<para>

A list of non-null element values most often appearing within values of

the expression. (Null for scalar types.)

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>most_common_elem_freqs</structfield> <type>float4[]</type>

</para>

<para>

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 <structfield>most_common_elems</structfield> is.)

</para></entry>

</row>

<row>

<entry role="catalog_table_entry"><para role="column_definition">

<structfield>elem_count_histogram</structfield> <type>float4[]</type>

</para>

<para>

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.)

</para></entry>

</row>

</tbody>

</tgroup>

</table>

<para>

The maximum number of entries in the array fields can be controlled on a

column-by-column basis using the <link linkend="sql-altertable"><command>ALTER

TABLE SET STATISTICS</command></link> command, or globally by setting the

<xref linkend="guc-default-statistics-target"/> run-time parameter.

</para>

</sect1>