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Add a new column to pg_am to specify whether an index AM supports backward

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scanning; GiST and GIN do not, and it seems like too much trouble to make
them do so.  By teaching ExecSupportsBackwardScan() about this restriction,
we ensure that the planner will protect a scroll cursor from the problem
by adding a Materialize node.

In passing, fix another longstanding bug in the same area: backwards scan of
a plan with set-returning functions in the targetlist did not work either,
since the TupFromTlist expansion code pays no attention to direction (and
has no way to run a SRF backwards anyway).  Again the fix is to make
ExecSupportsBackwardScan check this restriction.

Also adjust the index AM API specification to note that mark/restore support
is unnecessary if the AM can't produce ordered output.
REL8_5_ALPHA1_BRANCH
Tom Lane 17 years ago
parent 2a64931c4b
commit e4fb8ff06a
5 changed files with 119 additions and 42 deletions
Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL
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  1. 9
      doc/src/sgml/catalogs.sgml
  2. 19
      doc/src/sgml/indexam.sgml
  3. 73
      src/backend/executor/execAmi.c
  4. 4
      src/include/catalog/catversion.h
  5. 56
      src/include/catalog/pg_am.h

9
doc/src/sgml/catalogs.sgml
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@ -1,4 +1,4 @@
<!-- $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
-->
@ -401,6 +401,13 @@
<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>

19
doc/src/sgml/indexam.sgml
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@ -1,4 +1,4 @@
<!-- $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>
@ -474,15 +474,20 @@ amrestrpos (IndexScanDesc scan);
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>

73
src/backend/executor/execAmi.c
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@ -6,7 +6,7 @@
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/backend/executor/execAmi.c,v 1.99 2008/10/04 21:56:53 tgl Exp $
* $PostgreSQL: pgsql/src/backend/executor/execAmi.c,v 1.100 2008/10/17 22:10:29 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -42,6 +42,12 @@
#include "executor/nodeValuesscan.h"
#include "executor/nodeCtescan.h"
#include "executor/nodeWorktablescan.h"
#include "nodes/nodeFuncs.h"
#include "utils/syscache.h"
static bool TargetListSupportsBackwardScan(List *targetlist);
static bool IndexSupportsBackwardScan(Oid indexid);
/*
@ -390,7 +396,8 @@ ExecSupportsBackwardScan(Plan *node)
{
case T_Result:
if (outerPlan(node) != NULL)
return ExecSupportsBackwardScan(outerPlan(node));
return ExecSupportsBackwardScan(outerPlan(node)) &&
TargetListSupportsBackwardScan(node->targetlist);
else
return false;
@ -403,29 +410,85 @@ ExecSupportsBackwardScan(Plan *node)
if (!ExecSupportsBackwardScan((Plan *) lfirst(l)))
return false;
}
/* need not check tlist because Append doesn't evaluate it */
return true;
}
case T_SeqScan:
case T_IndexScan:
case T_TidScan:
case T_FunctionScan:
case T_ValuesScan:
case T_CteScan:
case T_WorkTableScan:
return true;
return TargetListSupportsBackwardScan(node->targetlist);
case T_IndexScan:
return IndexSupportsBackwardScan(((IndexScan *) node)->indexid) &&
TargetListSupportsBackwardScan(node->targetlist);
case T_SubqueryScan:
return ExecSupportsBackwardScan(((SubqueryScan *) node)->subplan);
return ExecSupportsBackwardScan(((SubqueryScan *) node)->subplan) &&
TargetListSupportsBackwardScan(node->targetlist);
case T_Material:
case T_Sort:
/* these don't evaluate tlist */
return true;
case T_Limit:
/* doesn't evaluate tlist */
return ExecSupportsBackwardScan(outerPlan(node));
default:
return false;
}
}
/*
* If the tlist contains set-returning functions, we can't support backward
* scan, because the TupFromTlist code is direction-ignorant.
*/
static bool
TargetListSupportsBackwardScan(List *targetlist)
{
if (expression_returns_set((Node *) targetlist))
return false;
return true;
}
/*
* An IndexScan node supports backward scan only if the index's AM does.
*/
static bool
IndexSupportsBackwardScan(Oid indexid)
{
bool result;
HeapTuple ht_idxrel;
HeapTuple ht_am;
Form_pg_class idxrelrec;
Form_pg_am amrec;
/* Fetch the pg_class tuple of the index relation */
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);
/* Fetch the pg_am tuple of the index' access method */
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);
return result;
}

4
src/include/catalog/catversion.h
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@ -37,7 +37,7 @@
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/catalog/catversion.h,v 1.498 2008/10/14 17:12:33 tgl Exp $
* $PostgreSQL: pgsql/src/include/catalog/catversion.h,v 1.499 2008/10/17 22:10:30 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -53,6 +53,6 @@
*/
/* yyyymmddN */
#define CATALOG_VERSION_NO 200810141
#define CATALOG_VERSION_NO 200810171
#endif

56
src/include/catalog/pg_am.h
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@ -8,7 +8,7 @@
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/catalog/pg_am.h,v 1.58 2008/09/15 18:43:41 tgl Exp $
* $PostgreSQL: pgsql/src/include/catalog/pg_am.h,v 1.59 2008/10/17 22:10:30 tgl Exp $
*
* NOTES
* the genbki.sh script reads this file and generates .bki
@ -41,6 +41,7 @@ CATALOG(pg_am,2601)
int2 amsupport; /* total number of support functions that this
* AM uses */
bool amcanorder; /* does AM support ordered scan results? */
bool amcanbackward; /* does AM support backward scan? */
bool amcanunique; /* does AM support UNIQUE indexes? */
bool amcanmulticol; /* does AM support multi-column indexes? */
bool amoptionalkey; /* can query omit key for the first column? */
@ -75,48 +76,49 @@ typedef FormData_pg_am *Form_pg_am;
* compiler constants for pg_am
* ----------------
*/
#define Natts_pg_am 25
#define Natts_pg_am 26
#define Anum_pg_am_amname 1
#define Anum_pg_am_amstrategies 2
#define Anum_pg_am_amsupport 3
#define Anum_pg_am_amcanorder 4
#define Anum_pg_am_amcanunique 5
#define Anum_pg_am_amcanmulticol 6
#define Anum_pg_am_amoptionalkey 7
#define Anum_pg_am_amindexnulls 8
#define Anum_pg_am_amsearchnulls 9
#define Anum_pg_am_amstorage 10
#define Anum_pg_am_amclusterable 11
#define Anum_pg_am_amkeytype 12
#define Anum_pg_am_aminsert 13
#define Anum_pg_am_ambeginscan 14
#define Anum_pg_am_amgettuple 15
#define Anum_pg_am_amgetbitmap 16
#define Anum_pg_am_amrescan 17
#define Anum_pg_am_amendscan 18
#define Anum_pg_am_ammarkpos 19
#define Anum_pg_am_amrestrpos 20
#define Anum_pg_am_ambuild 21
#define Anum_pg_am_ambulkdelete 22
#define Anum_pg_am_amvacuumcleanup 23
#define Anum_pg_am_amcostestimate 24
#define Anum_pg_am_amoptions 25
#define Anum_pg_am_amcanbackward 5
#define Anum_pg_am_amcanunique 6
#define Anum_pg_am_amcanmulticol 7
#define Anum_pg_am_amoptionalkey 8
#define Anum_pg_am_amindexnulls 9
#define Anum_pg_am_amsearchnulls 10
#define Anum_pg_am_amstorage 11
#define Anum_pg_am_amclusterable 12
#define Anum_pg_am_amkeytype 13
#define Anum_pg_am_aminsert 14
#define Anum_pg_am_ambeginscan 15
#define Anum_pg_am_amgettuple 16
#define Anum_pg_am_amgetbitmap 17
#define Anum_pg_am_amrescan 18
#define Anum_pg_am_amendscan 19
#define Anum_pg_am_ammarkpos 20
#define Anum_pg_am_amrestrpos 21
#define Anum_pg_am_ambuild 22
#define Anum_pg_am_ambulkdelete 23
#define Anum_pg_am_amvacuumcleanup 24
#define Anum_pg_am_amcostestimate 25
#define Anum_pg_am_amoptions 26
/* ----------------
* initial contents of pg_am
* ----------------
*/
DATA(insert OID = 403 ( btree 5 1 t t t t t t f t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbulkdelete btvacuumcleanup btcostestimate btoptions ));
DATA(insert OID = 403 ( btree 5 1 t t t t t t t f t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbulkdelete btvacuumcleanup btcostestimate btoptions ));
DESCR("b-tree index access method");
#define BTREE_AM_OID 403
DATA(insert OID = 405 ( hash 1 1 f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbulkdelete hashvacuumcleanup hashcostestimate hashoptions ));
DATA(insert OID = 405 ( hash 1 1 f t f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbulkdelete hashvacuumcleanup hashcostestimate hashoptions ));
DESCR("hash index access method");
#define HASH_AM_OID 405
DATA(insert OID = 783 ( gist 0 7 f f t t t t t t 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbulkdelete gistvacuumcleanup gistcostestimate gistoptions ));
DATA(insert OID = 783 ( gist 0 7 f f f t t t t t t 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbulkdelete gistvacuumcleanup gistcostestimate gistoptions ));
DESCR("GiST index access method");
#define GIST_AM_OID 783
DATA(insert OID = 2742 ( gin 0 5 f f t t f f t f 0 gininsert ginbeginscan gingettuple gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbulkdelete ginvacuumcleanup gincostestimate ginoptions ));
DATA(insert OID = 2742 ( gin 0 5 f f f t t f f t f 0 gininsert ginbeginscan gingettuple gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbulkdelete ginvacuumcleanup gincostestimate ginoptions ));
DESCR("GIN index access method");
#define GIN_AM_OID 2742

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