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Documentation of the system catalogs, directed toward PostgreSQL developers

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</entry>

</row>

<row>

<entry><structfield>attdistinct</structfield></entry>

<entry><type>float4</type></entry>

<entry></entry>

<entry>

<structfield>attdistinct</structfield>, if nonzero, is a user-specified

number-of-distinct-values figure to be used instead of estimating the

number of distinct values during <command>ANALYZE</>. Nonzero values

have the same meanings as for

<link linkend="catalog-pg-statistic"><structname>pg_statistic</></link>.<structfield>stadistinct</>

</entry>

</row>

<row>

<entry><structfield>attlen</structfield></entry>

<entry><type>int2</type></entry>

<entry></entry>

<entry>The number of distinct nonnull data values in the column.

A value greater than zero is the actual number of distinct values.

A value less than zero is the negative of a fraction of the number

of rows in the table (for example, a column in which values appear about

twice on the average could be represented by <structfield>stadistinct</> = -0.5).

A value less than zero is the negative of a multiplier for the number

of rows in the table; for example, a column in which values appear about

twice on the average could be represented by

<structfield>stadistinct</> = -0.5.

A zero value means the number of distinct values is unknown

</entry>

</row>

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$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.107 2009/07/20 02:42:27 adunstan Exp $

$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.108 2009/08/02 22:14:51 tgl Exp $

PostgreSQL documentation

-->

ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> DROP DEFAULT

ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET | DROP } NOT NULL

ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STATISTICS <replaceable class="PARAMETER">integer</replaceable>

ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STATISTICS DISTINCT <replaceable class="PARAMETER">number</replaceable>

ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }

ADD <replaceable class="PARAMETER">table_constraint</replaceable>

DROP CONSTRAINT [ IF EXISTS ] <replaceable class="PARAMETER">constraint_name</replaceable> [ RESTRICT | CASCADE ]

dropped as well. You will need to say <literal>CASCADE</> if

anything outside the table depends on the column, for example,

foreign key references or views.

If <literal>IF EXISTS</literal> is specified and the column

does not exist, no error is thrown. In this case a notice

If <literal>IF EXISTS</literal> is specified and the column

does not exist, no error is thrown. In this case a notice

is issued instead.

</para>

</listitem>

</listitem>

</varlistentry>

<varlistentry>

<term><literal>SET STATISTICS DISTINCT</literal></term>

<listitem>

<para>

This form overrides the number-of-distinct-values estimate made by

subsequent <xref linkend="sql-analyze" endterm="sql-analyze-title">

operations. When set to a positive value, <command>ANALYZE</> will

assume that the column contains exactly the specified number of distinct

nonnull values. When set to a negative value, which must be greater

than or equal to -1, <command>ANALYZE</> will assume that the number of

distinct nonnull values in the column is linear in the size of the

table; the exact count is to be computed by multiplying the estimated

table size by the absolute value of the given number. For example,

a value of -1 implies that all values in the column are distinct, while

a value of -0.5 implies that each value appears twice on the average.

This can be useful when the size of the table changes over time, since

the multiplication by the number of rows in the table is not performed

until query planning time. Specify a value of 0 to revert to estimating

the number of distinct values normally. For more information on the use

of statistics by the <productname>PostgreSQL</productname> query

planner, refer to <xref linkend="planner-stats">.

</para>

</listitem>

</varlistentry>

<varlistentry>

<indexterm>

<primary>TOAST</primary>

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PostgreSQL documentation

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the target causes a proportional increase in the time and space needed

to do <command>ANALYZE</command>.

</para>

<para>

One of the values estimated by <command>ANALYZE</command> is the number of

distinct values that appear in each column. Because only a subset of the

rows are examined, this estimate can sometimes be quite inaccurate, even

with the largest possible statistics target. If this inaccuracy leads to

bad query plans, a more accurate value can be determined manually and then

installed with

<command>ALTER TABLE ... ALTER COLUMN ... SET STATISTICS DISTINCT</>

(see <xref linkend="sql-altertable" endterm="sql-altertable-title">).

</para>

</refsect1>

<refsect1>

return false;

if (attr1->attstattarget!=attr2->attstattarget)

return false;

if (attr1->attdistinct!=attr2->attdistinct)

return false;

if (attr1->attlen!=attr2->attlen)

return false;

if (attr1->attndims!=attr2->attndims)

MemSet(NameStr(att->attname),0,NAMEDATALEN);

att->attstattarget=-1;

att->attdistinct=0;

att->attcacheoff=-1;

att->atttypmod=typmod;

}

attrtypes[attnum]->attstattarget=-1;

attrtypes[attnum]->attdistinct=0;

attrtypes[attnum]->attcacheoff=-1;

attrtypes[attnum]->atttypmod=-1;

attrtypes[attnum]->attislocal= true;

staticFormData_pg_attributea1= {

0, {"ctid"},TIDOID,0,sizeof(ItemPointerData),

0, {"ctid"},TIDOID,0,0,sizeof(ItemPointerData),

SelfItemPointerAttributeNumber,0,-1,-1,

false,'p','s', true, false, false, true,0, {0}

};

staticFormData_pg_attributea2= {

0, {"oid"},OIDOID,0,sizeof(Oid),

0, {"oid"},OIDOID,0,0,sizeof(Oid),

ObjectIdAttributeNumber,0,-1,-1,

true,'p','i', true, false, false, true,0, {0}

};

staticFormData_pg_attributea3= {

0, {"xmin"},XIDOID,0,sizeof(TransactionId),

0, {"xmin"},XIDOID,0,0,sizeof(TransactionId),

MinTransactionIdAttributeNumber,0,-1,-1,

true,'p','i', true, false, false, true,0, {0}

};

staticFormData_pg_attributea4= {

0, {"cmin"},CIDOID,0,sizeof(CommandId),

0, {"cmin"},CIDOID,0,0,sizeof(CommandId),

MinCommandIdAttributeNumber,0,-1,-1,

true,'p','i', true, false, false, true,0, {0}

};

staticFormData_pg_attributea5= {

0, {"xmax"},XIDOID,0,sizeof(TransactionId),

0, {"xmax"},XIDOID,0,0,sizeof(TransactionId),

MaxTransactionIdAttributeNumber,0,-1,-1,

true,'p','i', true, false, false, true,0, {0}

};

staticFormData_pg_attributea6= {

0, {"cmax"},CIDOID,0,sizeof(CommandId),

0, {"cmax"},CIDOID,0,0,sizeof(CommandId),

MaxCommandIdAttributeNumber,0,-1,-1,

true,'p','i', true, false, false, true,0, {0}

};

staticFormData_pg_attributea7= {

0, {"tableoid"},OIDOID,0,sizeof(Oid),

0, {"tableoid"},OIDOID,0,0,sizeof(Oid),

TableOidAttributeNumber,0,-1,-1,

true,'p','i', true, false, false, true,0, {0}

};

values[Anum_pg_attribute_attname-1]=NameGetDatum(&new_attribute->attname);

values[Anum_pg_attribute_atttypid-1]=ObjectIdGetDatum(new_attribute->atttypid);

values[Anum_pg_attribute_attstattarget-1]=Int32GetDatum(new_attribute->attstattarget);

values[Anum_pg_attribute_attdistinct-1]=Float4GetDatum(new_attribute->attdistinct);

values[Anum_pg_attribute_attlen-1]=Int16GetDatum(new_attribute->attlen);

values[Anum_pg_attribute_attnum-1]=Int16GetDatum(new_attribute->attnum);

values[Anum_pg_attribute_attndims-1]=Int32GetDatum(new_attribute->attndims);

attr->attrelid=new_rel_oid;

attr->attstattarget=-1;

attr->attdistinct=0;

attr->attcacheoff=-1;

InsertPgAttributeTuple(rel,attr,indstate);

to->attnum=i+1;

to->attstattarget=-1;

to->attdistinct=0;

to->attcacheoff=-1;

to->attnotnull= false;

to->atthasdef= false;

std_fetch_func,

numrows,

totalrows);

if (stats->attr->attdistinct!=0)

stats->stadistinct=stats->attr->attdistinct;

MemoryContextResetAndDeleteChildren(col_context);

}

ind_fetch_func,

numindexrows,

totalindexrows);

if (stats->attr->attdistinct!=0)

stats->stadistinct=stats->attr->attdistinct;

MemoryContextResetAndDeleteChildren(col_context);

}

}