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><H1
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><A
NAME="CATALOG-PG-ATTRIBUTE"
>44.7. <TT
CLASS="STRUCTNAME"
>pg_attribute</TT
></A
></H1
><A
NAME="AEN68513"
></A
><P
> The catalog <TT
CLASS="STRUCTNAME"
>pg_attribute</TT
> stores information about
table columns. There will be exactly one
<TT
CLASS="STRUCTNAME"
>pg_attribute</TT
> row for every column in every
table in the database. (There will also be attribute entries for
indexes, and indeed all objects that have <TT
CLASS="STRUCTNAME"
>pg_class</TT
>
entries.)
</P
><P
> The term attribute is equivalent to column and is used for
historical reasons.
</P
><DIV
CLASS="TABLE"
><A
NAME="AEN68520"
></A
><P
><B
>Table 44-7. <TT
CLASS="STRUCTNAME"
>pg_attribute</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"
>attrelid</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-class.html"
><TT
CLASS="STRUCTNAME"
>pg_class</TT
></A
>.oid</TT
></TD
><TD
>The table this column belongs to</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attname</TT
></TD
><TD
><TT
CLASS="TYPE"
>name</TT
></TD
><TD
> </TD
><TD
>The column name</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>atttypid</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-type.html"
><TT
CLASS="STRUCTNAME"
>pg_type</TT
></A
>.oid</TT
></TD
><TD
>The data type of this column</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attstattarget</TT
></TD
><TD
><TT
CLASS="TYPE"
>int4</TT
></TD
><TD
> </TD
><TD
> <TT
CLASS="STRUCTFIELD"
>attstattarget</TT
> controls the level of detail
of statistics accumulated for this column by
<A
HREF="sql-analyze.html"
><I
>ANALYZE</I
></A
>.
A zero value indicates that no statistics should be collected.
A negative value says to use the system default statistics target.
The exact meaning of positive values is data type-dependent.
For scalar data types, <TT
CLASS="STRUCTFIELD"
>attstattarget</TT
>
is both the target number of <SPAN
CLASS="QUOTE"
>"most common values"</SPAN
>
to collect, and the target number of histogram bins to create
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attlen</TT
></TD
><TD
><TT
CLASS="TYPE"
>int2</TT
></TD
><TD
> </TD
><TD
> A copy of <TT
CLASS="LITERAL"
>pg_type.typlen</TT
> of this column's
type
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attnum</TT
></TD
><TD
><TT
CLASS="TYPE"
>int2</TT
></TD
><TD
> </TD
><TD
> The number of the column. Ordinary columns are numbered from 1
up. System columns, such as <TT
CLASS="STRUCTFIELD"
>oid</TT
>,
have (arbitrary) negative numbers
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attndims</TT
></TD
><TD
><TT
CLASS="TYPE"
>int4</TT
></TD
><TD
> </TD
><TD
> Number of dimensions, if the column is an array type; otherwise 0.
(Presently, the number of dimensions of an array is not enforced,
so any nonzero value effectively means <SPAN
CLASS="QUOTE"
>"it's an array"</SPAN
>)
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attcacheoff</TT
></TD
><TD
><TT
CLASS="TYPE"
>int4</TT
></TD
><TD
> </TD
><TD
> Always -1 in storage, but when loaded into a row descriptor
in memory this might be updated to cache the offset of the attribute
within the row
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>atttypmod</TT
></TD
><TD
><TT
CLASS="TYPE"
>int4</TT
></TD
><TD
> </TD
><TD
> <TT
CLASS="STRUCTFIELD"
>atttypmod</TT
> records type-specific data
supplied at table creation time (for example, the maximum
length of a <TT
CLASS="TYPE"
>varchar</TT
> column). It is passed to
type-specific input functions and length coercion functions.
The value will generally be -1 for types that do not need <TT
CLASS="STRUCTFIELD"
>atttypmod</TT
>
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attbyval</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
> </TD
><TD
> A copy of <TT
CLASS="LITERAL"
>pg_type.typbyval</TT
> of this column's type
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attstorage</TT
></TD
><TD
><TT
CLASS="TYPE"
>char</TT
></TD
><TD
> </TD
><TD
> Normally a copy of <TT
CLASS="LITERAL"
>pg_type.typstorage</TT
> of this
column's type. For TOAST-able data types, this can be altered
after column creation to control storage policy
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attalign</TT
></TD
><TD
><TT
CLASS="TYPE"
>char</TT
></TD
><TD
> </TD
><TD
> A copy of <TT
CLASS="LITERAL"
>pg_type.typalign</TT
> of this column's type
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attnotnull</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
> </TD
><TD
> This represents a not-null constraint. It is possible to
change this column to enable or disable the constraint
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>atthasdef</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
> </TD
><TD
> This column has a default value, in which case there will be a
corresponding entry in the <TT
CLASS="STRUCTNAME"
>pg_attrdef</TT
>
catalog that actually defines the value
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attisdropped</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
> </TD
><TD
> This column has been dropped and is no longer valid. A dropped
column is still physically present in the table, but is
ignored by the parser and so cannot be accessed via SQL
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attislocal</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
> </TD
><TD
> This column is defined locally in the relation. Note that a column can
be locally defined and inherited simultaneously
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>attinhcount</TT
></TD
><TD
><TT
CLASS="TYPE"
>int4</TT
></TD
><TD
> </TD
><TD
> The number of direct ancestors this column has. A column with a
nonzero number of ancestors cannot be dropped nor renamed
</TD
></TR
></TBODY
></TABLE
></DIV
><P
> In a dropped column's <TT
CLASS="STRUCTNAME"
>pg_attribute</TT
> entry,
<TT
CLASS="STRUCTFIELD"
>atttypid</TT
> is reset to zero, but
<TT
CLASS="STRUCTFIELD"
>attlen</TT
> and the other fields copied from
<TT
CLASS="STRUCTNAME"
>pg_type</TT
> are still valid. This arrangement is needed
to cope with the situation where the dropped column's data type was
later dropped, and so there is no <TT
CLASS="STRUCTNAME"
>pg_type</TT
> row anymore.
<TT
CLASS="STRUCTFIELD"
>attlen</TT
> and the other fields can be used
to interpret the contents of a row of the table.
</P
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