<!--

Documentation of the system catalogs, directed toward PostgreSQL developers

$PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.83 2004/02/12 23:41:00 tgl Exp $

$PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.84 2004/02/15 21:01:38 tgl Exp $

-->

<chapter id="catalogs">

</indexterm>

<para>

The catalog <structname>pg_statistic</structname> stores statistical data about

the contents of the database. Entries are created by

The catalog <structname>pg_statistic</structname> stores statistical data

aboutthe contents of the database. Entries are created by

<command>ANALYZE</command> 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 up-to-date.

</para>

<para>

<structname>pg_statistic</structname> also stores statistical data about

the values of index expressions. These are described as if they were

actual data columns; in particular, <structfield>starelid</structfield>

references the index. No entry is made for an ordinary non-expression

index column, however, since it would be redundant with the entry

for the underlying table column.

</para>

<para>

Since different kinds of statistics may be appropriate for different

kinds of data, <structname>pg_statistic</structname> 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 <structname>pg_statistic</structname>. Everything else

is stored in <quote>slots</quote>, which are groups of associated columns whose

content is identified by a code number in one of the slot's columns.

is stored in <quote>slots</quote>, which are groups of associated columns

whosecontent is identified by a code number in one of the slot's columns.

For more information see

<filename>src/include/catalog/pg_statistic.h</filename>.

</para>

<entry><structfield>starelid</structfield></entry>

<entry><type>oid</type></entry>

<entry><literal><link linkend="catalog-pg-class"><structname>pg_class</structname></link>.oid</literal></entry>

<entry>The table that the described column belongs to</entry>

<entry>The tableor indexthat the described column belongs to</entry>

</row>

<row>

staticvoidStoreRelCheck(Relationrel,char*ccname,char*ccbin);

staticvoidStoreConstraints(Relationrel,TupleDesctupdesc);

staticvoidSetRelationNumChecks(Relationrel,intnumchecks);

staticvoidRemoveStatistics(Relationrel,AttrNumberattnum);

RemoveStatistics(Relationrel,AttrNumberattnum)

{

Relationpgstatistic;

to->attnum=i+1;

to->attstattarget=0;

to->attstattarget=-1;

to->attcacheoff=-1;

to->attnotnull= false;

to->atthasdef= false;

to->attbyval=typeTup->typbyval;

to->attstorage=typeTup->typstorage;

to->attalign=typeTup->typalign;

to->attstattarget=-1;

to->attcacheoff=-1;

to->atttypmod=-1;

to->attislocal= true;

RelationuserIndexRelation;

RelationindexRelation;

HeapTupletuple;

boolhasexprs;

inti;

Assert(OidIsValid(indexId));

DeleteAttributeTuples(indexId);

indexRelation=heap_openr(IndexRelationName,RowExclusiveLock);

if (!HeapTupleIsValid(tuple))

elog(ERROR,"cache lookup failed for index %u",indexId);

hasexprs= !heap_attisnull(tuple,Anum_pg_index_indexprs);

simple_heap_delete(indexRelation,&tuple->t_self);

ReleaseSysCache(tuple);

heap_close(indexRelation,RowExclusiveLock);

if (hasexprs)

RemoveStatistics(userIndexRelation,0);