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>PostgreSQL 8.0.11 Documentation</TH
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="CATALOG-PG-AM"
>41.3. <TT
CLASS="STRUCTNAME"
>pg_am</TT
></A
></H1
><A
NAME="AEN52072"
></A
><P
> The catalog <TT
CLASS="STRUCTNAME"
>pg_am</TT
> stores information about index access
methods. There is one row for each index access method supported by
the system.
</P
><DIV
CLASS="TABLE"
><A
NAME="AEN52076"
></A
><P
><B
>Table 41-3. <TT
CLASS="STRUCTNAME"
>pg_am</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"
>amname</TT
></TD
><TD
><TT
CLASS="TYPE"
>name</TT
></TD
><TD
> </TD
><TD
>Name of the access method</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amowner</TT
></TD
><TD
><TT
CLASS="TYPE"
>int4</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-shadow.html"
><TT
CLASS="STRUCTNAME"
>pg_shadow</TT
></A
>.usesysid</TT
></TD
><TD
>User ID of the owner (currently not used)</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amstrategies</TT
></TD
><TD
><TT
CLASS="TYPE"
>int2</TT
></TD
><TD
> </TD
><TD
>Number of operator strategies for this access method</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amsupport</TT
></TD
><TD
><TT
CLASS="TYPE"
>int2</TT
></TD
><TD
> </TD
><TD
>Number of support routines for this access method</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amorderstrategy</TT
></TD
><TD
><TT
CLASS="TYPE"
>int2</TT
></TD
><TD
> </TD
><TD
>Zero if the index offers no sort order, otherwise the strategy
number of the strategy operator that describes the sort order</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amcanunique</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
> </TD
><TD
>Does the access method support unique indexes?</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amcanmulticol</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
> </TD
><TD
>Does the access method support multicolumn indexes?</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amindexnulls</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
> </TD
><TD
>Does the access method support null index entries?</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amconcurrent</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
> </TD
><TD
>Does the access method support concurrent updates?</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amgettuple</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
><SPAN
CLASS="QUOTE"
>"Next valid tuple"</SPAN
> function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>aminsert</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
><SPAN
CLASS="QUOTE"
>"Insert this tuple"</SPAN
> function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>ambeginscan</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
><SPAN
CLASS="QUOTE"
>"Start new scan"</SPAN
> function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amrescan</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
><SPAN
CLASS="QUOTE"
>"Restart this scan"</SPAN
> function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amendscan</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
><SPAN
CLASS="QUOTE"
>"End this scan"</SPAN
> function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>ammarkpos</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
><SPAN
CLASS="QUOTE"
>"Mark current scan position"</SPAN
> function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amrestrpos</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
><SPAN
CLASS="QUOTE"
>"Restore marked scan position"</SPAN
> function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>ambuild</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
><SPAN
CLASS="QUOTE"
>"Build new index"</SPAN
> function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>ambulkdelete</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
>Bulk-delete function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amvacuumcleanup</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
>Post-<TT
CLASS="COMMAND"
>VACUUM</TT
> cleanup function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>amcostestimate</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
>Function to estimate cost of an index scan</TD
></TR
></TBODY
></TABLE
></DIV
><P
> An index access method that supports multiple columns (has
<TT
CLASS="STRUCTFIELD"
>amcanmulticol</TT
> true) <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>must</I
></SPAN
>
support indexing null values in columns after the first, because the planner
will assume the index can be used for queries on just the first
column(s). For example, consider an index on (a,b) and a query with
<TT
CLASS="LITERAL"
>WHERE a = 4</TT
>. The system will assume the index can be used to scan for
rows with <TT
CLASS="LITERAL"
>a = 4</TT
>, which is wrong if the index omits rows where <TT
CLASS="LITERAL"
>b</TT
> is null.
It is, however, OK to omit rows where the first indexed column is null.
(GiST currently does so.)
<TT
CLASS="STRUCTFIELD"
>amindexnulls</TT
> should be set true only if the
index access method indexes all rows, including arbitrary combinations of null values.
</P
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