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>PostgreSQL 8.0.11 Documentation</TH
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>Chapter 41. System Catalogs</TD
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><H1
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><A
NAME="VIEW-PG-STATS"
>41.36. <TT
CLASS="STRUCTNAME"
>pg_stats</TT
></A
></H1
><A
NAME="AEN55470"
></A
><P
> The view <TT
CLASS="STRUCTNAME"
>pg_stats</TT
> provides access to
the information stored in the <A
HREF="catalog-pg-statistic.html"
><TT
CLASS="STRUCTNAME"
>pg_statistic</TT
></A
>
catalog. This view allows access only to rows of
<TT
CLASS="STRUCTNAME"
>pg_statistic</TT
> that correspond to tables the
user has permission to read, and therefore it is safe to allow public
read access to this view.
</P
><P
> <TT
CLASS="STRUCTNAME"
>pg_stats</TT
> 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 <TT
CLASS="STRUCTNAME"
>pg_statistic</TT
>.
</P
><DIV
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><P
><B
>Table 41-36. <TT
CLASS="STRUCTNAME"
>pg_stats</TT
> Columns</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Type</TH
><TH
>References</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>schemaname</TT
></TD
><TD
><TT
CLASS="TYPE"
>name</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-namespace.html"
><TT
CLASS="STRUCTNAME"
>pg_namespace</TT
></A
>.nspname</TT
></TD
><TD
>name of schema containing table</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>tablename</TT
></TD
><TD
><TT
CLASS="TYPE"
>name</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-class.html"
><TT
CLASS="STRUCTNAME"
>pg_class</TT
></A
>.relname</TT
></TD
><TD
>name of table</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attname</TT
></TD
><TD
><TT
CLASS="TYPE"
>name</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-attribute.html"
><TT
CLASS="STRUCTNAME"
>pg_attribute</TT
></A
>.attname</TT
></TD
><TD
>name of the column described by this row</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>null_frac</TT
></TD
><TD
><TT
CLASS="TYPE"
>real</TT
></TD
><TD
> </TD
><TD
>fraction of column entries that are null</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>avg_width</TT
></TD
><TD
><TT
CLASS="TYPE"
>integer</TT
></TD
><TD
> </TD
><TD
>average width in bytes of column's entries</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>n_distinct</TT
></TD
><TD
><TT
CLASS="TYPE"
>real</TT
></TD
><TD
> </TD
><TD
>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 <TT
CLASS="COMMAND"
>ANALYZE</TT
> 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.
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>most_common_vals</TT
></TD
><TD
><TT
CLASS="TYPE"
>anyarray</TT
></TD
><TD
> </TD
><TD
>A list of the most common values in the column. (NULL if
no values seem to be more common than any others.)</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>most_common_freqs</TT
></TD
><TD
><TT
CLASS="TYPE"
>real[]</TT
></TD
><TD
> </TD
><TD
>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 <TT
CLASS="STRUCTFIELD"
>most_common_vals</TT
> is.)
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>histogram_bounds</TT
></TD
><TD
><TT
CLASS="TYPE"
>anyarray</TT
></TD
><TD
> </TD
><TD
>A list of values that divide the column's values into
groups of approximately equal population. The values in
<TT
CLASS="STRUCTFIELD"
>most_common_vals</TT
>, if present, are omitted from this
histogram calculation. (This column is NULL if the column data type
does not have a <TT
CLASS="LITERAL"
><</TT
> operator or if the
<TT
CLASS="STRUCTFIELD"
>most_common_vals</TT
> list accounts for the entire
population.)
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>correlation</TT
></TD
><TD
><TT
CLASS="TYPE"
>real</TT
></TD
><TD
> </TD
><TD
>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 <TT
CLASS="LITERAL"
><</TT
> operator.)
</TD
></TR
></TBODY
></TABLE
></DIV
><P
> The maximum number of entries in the <TT
CLASS="STRUCTFIELD"
>most_common_vals</TT
>
and <TT
CLASS="STRUCTFIELD"
>histogram_bounds</TT
> arrays can be set on a
column-by-column basis using the <TT
CLASS="COMMAND"
>ALTER TABLE SET STATISTICS</TT
>
command, or globally by setting the
<A
HREF="runtime-config.html#GUC-DEFAULT-STATISTICS-TARGET"
>default_statistics_target</A
> runtime parameter.
</P
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