<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.178 2008/10/06 13:59:37 tgl Exp $ -->

<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.179 2008/10/17 22:10:29 tgl Exp $ -->

<!--

Documentation of the system catalogs, directed toward PostgreSQL developers

-->

<entry>Does the access method support ordered scans?</entry>

</row>

<row>

<entry><structfield>amcanbackward</structfield></entry>

<entry><type>bool</type></entry>

<entry></entry>

<entry>Does the access method support backward scanning?</entry>

</row>

<row>

<entry><structfield>amcanunique</structfield></entry>

<entry><type>bool</type></entry>

<!-- $PostgreSQL: pgsql/doc/src/sgml/indexam.sgml,v 2.27 2008/08/14 18:47:58 tgl Exp $ -->

<!-- $PostgreSQL: pgsql/doc/src/sgml/indexam.sgml,v 2.28 2008/10/17 22:10:29 tgl Exp $ -->

<chapter id="indexam">

<title>Index Access Method Interface Definition</title>

normally would. (This will only occur for access

methods that advertise they support ordered scans.) After the

first call, <function>amgettuple</> must be prepared to advance the scan in

either direction from the most recently returned entry.

either direction from the most recently returned entry. (But if

<structname>pg_am</>.<structfield>amcanbackward</> is false, all subsequent

calls will have the same direction as the first one.)

</para>

<para>

The access method must support <quote>marking</> a position in a scan

and later returning to the marked position. The same position might be

restored multiple times. However, only one position need be remembered

per scan; a new <function>ammarkpos</> call overrides the previously

marked position.

Access methods that support ordered scans must support <quote>marking</> a

position in a scan and later returning to the marked position. The same

position might be restored multiple times. However, only one position need

be remembered per scan; a new <function>ammarkpos</> call overrides the

previously marked position. An access method that does not support

ordered scans should still provide mark and restore functions in

<structname>pg_am</>, but it is sufficient to have them throw errors if

called.

</para>

<para>

staticboolTargetListSupportsBackwardScan(List*targetlist);

staticboolIndexSupportsBackwardScan(Oidindexid);

{

caseT_Result:

if (outerPlan(node)!=NULL)

returnExecSupportsBackwardScan(outerPlan(node));

returnExecSupportsBackwardScan(outerPlan(node))&&

TargetListSupportsBackwardScan(node->targetlist);

return false;

if (!ExecSupportsBackwardScan((Plan*)lfirst(l)))

return false;

}

return true;

}

caseT_SeqScan:

caseT_IndexScan:

caseT_TidScan:

caseT_FunctionScan:

caseT_ValuesScan:

caseT_CteScan:

caseT_WorkTableScan:

return true;

returnTargetListSupportsBackwardScan(node->targetlist);

caseT_IndexScan:

returnIndexSupportsBackwardScan(((IndexScan*)node)->indexid)&&

TargetListSupportsBackwardScan(node->targetlist);

caseT_SubqueryScan:

returnExecSupportsBackwardScan(((SubqueryScan*)node)->subplan);

returnExecSupportsBackwardScan(((SubqueryScan*)node)->subplan)&&

TargetListSupportsBackwardScan(node->targetlist);

caseT_Material:

caseT_Sort:

return true;

caseT_Limit:

returnExecSupportsBackwardScan(outerPlan(node));

default:

return false;

}

}

TargetListSupportsBackwardScan(List*targetlist)

{

if (expression_returns_set((Node*)targetlist))

return false;

return true;

}

IndexSupportsBackwardScan(Oidindexid)

{

boolresult;

HeapTupleht_idxrel;

HeapTupleht_am;

Form_pg_classidxrelrec;

Form_pg_amamrec;

ht_idxrel=SearchSysCache(RELOID,

ObjectIdGetDatum(indexid),

0,0,0);

if (!HeapTupleIsValid(ht_idxrel))

elog(ERROR,"cache lookup failed for relation %u",indexid);

idxrelrec= (Form_pg_class)GETSTRUCT(ht_idxrel);

ht_am=SearchSysCache(AMOID,

ObjectIdGetDatum(idxrelrec->relam),

0,0,0);

if (!HeapTupleIsValid(ht_am))

elog(ERROR,"cache lookup failed for access method %u",

idxrelrec->relam);

amrec= (Form_pg_am)GETSTRUCT(ht_am);

result=amrec->amcanbackward;

ReleaseSysCache(ht_idxrel);

ReleaseSysCache(ht_am);

returnresult;

}

int2amsupport;/* total number of support functions that this

boolamcanorder;/* does AM support ordered scan results? */

boolamcanbackward;/* does AM support backward scan? */

boolamcanunique;/* does AM support UNIQUE indexes? */

boolamcanmulticol;/* does AM support multi-column indexes? */

boolamoptionalkey;/* can query omit key for the first column? */

#defineAnum_pg_am_amname1

#defineAnum_pg_am_amstrategies2

#defineAnum_pg_am_amsupport3

#defineAnum_pg_am_amcanorder4

#defineAnum_pg_am_amcanunique5

#defineAnum_pg_am_amcanmulticol6

#defineAnum_pg_am_amoptionalkey7

#defineAnum_pg_am_amindexnulls8

#defineAnum_pg_am_amsearchnulls9

#defineAnum_pg_am_amstorage10

#defineAnum_pg_am_amclusterable11

#defineAnum_pg_am_amkeytype12

#defineAnum_pg_am_aminsert13

#defineAnum_pg_am_ambeginscan14

#defineAnum_pg_am_amgettuple15

#defineAnum_pg_am_amgetbitmap16

#defineAnum_pg_am_amrescan17

#defineAnum_pg_am_amendscan18

#defineAnum_pg_am_ammarkpos19

#defineAnum_pg_am_amrestrpos20

#defineAnum_pg_am_ambuild21

#defineAnum_pg_am_ambulkdelete22

#defineAnum_pg_am_amvacuumcleanup23

#defineAnum_pg_am_amcostestimate24

#defineAnum_pg_am_amoptions25

#defineAnum_pg_am_amcanbackward5

#defineAnum_pg_am_amcanunique6

#defineAnum_pg_am_amcanmulticol7

#defineAnum_pg_am_amoptionalkey8

#defineAnum_pg_am_amindexnulls9

#defineAnum_pg_am_amsearchnulls10

#defineAnum_pg_am_amstorage11

#defineAnum_pg_am_amclusterable12

#defineAnum_pg_am_amkeytype13

#defineAnum_pg_am_aminsert14

#defineAnum_pg_am_ambeginscan15

#defineAnum_pg_am_amgettuple16

#defineAnum_pg_am_amgetbitmap17

#defineAnum_pg_am_amrescan18

#defineAnum_pg_am_amendscan19

#defineAnum_pg_am_ammarkpos20

#defineAnum_pg_am_amrestrpos21

#defineAnum_pg_am_ambuild22

#defineAnum_pg_am_ambulkdelete23

#defineAnum_pg_am_amvacuumcleanup24

#defineAnum_pg_am_amcostestimate25

#defineAnum_pg_am_amoptions26

DATA(insertOID=403 (btree51ttttttft0btinsertbtbeginscanbtgettuplebtgetbitmapbtrescanbtendscanbtmarkposbtrestrposbtbuildbtbulkdeletebtvacuumcleanupbtcostestimatebtoptions ));

DATA(insertOID=403 (btree51tttttttft0btinsertbtbeginscanbtgettuplebtgetbitmapbtrescanbtendscanbtmarkposbtrestrposbtbuildbtbulkdeletebtvacuumcleanupbtcostestimatebtoptions ));

DESCR("b-tree index access method");

#defineBTREE_AM_OID 403

DATA(insertOID=405 (hash11ffffffff23hashinserthashbeginscanhashgettuplehashgetbitmaphashrescanhashendscanhashmarkposhashrestrposhashbuildhashbulkdeletehashvacuumcleanuphashcostestimatehashoptions ));

DATA(insertOID=405 (hash11ftfffffff23hashinserthashbeginscanhashgettuplehashgetbitmaphashrescanhashendscanhashmarkposhashrestrposhashbuildhashbulkdeletehashvacuumcleanuphashcostestimatehashoptions ));

DESCR("hash index access method");

#defineHASH_AM_OID 405

DATA(insertOID=783 (gist07fftttttt0gistinsertgistbeginscangistgettuplegistgetbitmapgistrescangistendscangistmarkposgistrestrposgistbuildgistbulkdeletegistvacuumcleanupgistcostestimategistoptions ));

DATA(insertOID=783 (gist07ffftttttt0gistinsertgistbeginscangistgettuplegistgetbitmapgistrescangistendscangistmarkposgistrestrposgistbuildgistbulkdeletegistvacuumcleanupgistcostestimategistoptions ));

DESCR("GiST index access method");

#defineGIST_AM_OID 783

DATA(insertOID=2742 (gin05ffttfftf0gininsertginbeginscangingettuplegingetbitmapginrescanginendscanginmarkposginrestrposginbuildginbulkdeleteginvacuumcleanupgincostestimateginoptions ));

DATA(insertOID=2742 (gin05fffttfftf0gininsertginbeginscangingettuplegingetbitmapginrescanginendscanginmarkposginrestrposginbuildginbulkdeleteginvacuumcleanupgincostestimateginoptions ));

DESCR("GIN index access method");

#defineGIN_AM_OID 2742