database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-07-15 19:21:59 +03:00
Code Activity

Support index-only scans using the visibility map to avoid heap fetches.

Browse Source 
When a btree index contains all columns required by the query, and the
visibility map shows that all tuples on a target heap page are
visible-to-all, we don't need to fetch that heap page.  This patch depends
on the previous patches that made the visibility map reliable.

There's a fair amount left to do here, notably trying to figure out a less
chintzy way of estimating the cost of an index-only scan, but the core
functionality seems ready to commit.

Robert Haas and Ibrar Ahmed, with some previous work by Heikki Linnakangas.
This commit is contained in:
Tom Lane
2011-10-07 20:13:02 -04:00
parent caa1054df8
commit a2822fb933
34 changed files with 704 additions and 203 deletions
Download Patch File Download Diff File Expand all files Collapse all files

89
src/backend/optimizer/path/indxpath.c
View File

@ -18,6 +18,7 @@
#include <math.h>
#include "access/skey.h"
#include "access/sysattr.h"
#include "catalog/pg_am.h"
#include "catalog/pg_collation.h"
#include "catalog/pg_operator.h"
@ -88,6 +89,7 @@ static PathClauseUsage *classify_index_clause_usage(Path *path,
List **clauselist);
static void find_indexpath_quals(Path *bitmapqual, List **quals, List **preds);
static int find_list_position(Node *node, List **nodelist);
static bool check_index_only(RelOptInfo *rel, IndexOptInfo *index);
static List *group_clauses_by_indexkey(IndexOptInfo *index,
List *clauses, List *outer_clauses,
Relids outer_relids,
@ -314,6 +316,8 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
bool useful_predicate;
bool found_clause;
bool index_is_ordered;
bool index_only_scan = false;
bool checked_index_only = false;
/*
* Check that index supports the desired scan type(s)
@ -438,6 +442,10 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
*/
if (found_clause || useful_pathkeys != NIL || useful_predicate)
{
/* First, detect whether index-only scan is possible */
index_only_scan = check_index_only(rel, index);
checked_index_only = true;
ipath = create_index_path(root, index,
restrictclauses,
orderbyclauses,
@ -445,6 +453,7 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
index_is_ordered ?
ForwardScanDirection :
NoMovementScanDirection,
index_only_scan,
outer_rel);
result = lappend(result, ipath);
}
@ -462,11 +471,15 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
index_pathkeys);
if (useful_pathkeys != NIL)
{
if (!checked_index_only)
index_only_scan = check_index_only(rel, index);
ipath = create_index_path(root, index,
restrictclauses,
NIL,
useful_pathkeys,
BackwardScanDirection,
index_only_scan,
outer_rel);
result = lappend(result, ipath);
}
@ -1040,6 +1053,82 @@ find_list_position(Node *node, List **nodelist)
}
/*
* check_index_only
* Determine whether an index-only scan is possible for this index.
*/
static bool
check_index_only(RelOptInfo *rel, IndexOptInfo *index)
{
bool result;
Bitmapset *attrs_used = NULL;
Bitmapset *index_attrs = NULL;
ListCell *lc;
int i;
/* Index-only scans must be enabled, and AM must be capable of it */
if (!enable_indexonlyscan)
return false;
if (!index->amcanreturn)
return false;
/*
* Check that all needed attributes of the relation are available from
* the index.
*
* XXX this is overly conservative for partial indexes, since we will
* consider attributes involved in the index predicate as required even
* though the predicate won't need to be checked at runtime. (The same
* is true for attributes used only in index quals, if we are certain
* that the index is not lossy.) However, it would be quite expensive
* to determine that accurately at this point, so for now we take the
* easy way out.
*/
/*
* Add all the attributes needed for joins or final output. Note: we must
* look at reltargetlist, not the attr_needed data, because attr_needed
* isn't computed for inheritance child rels.
*/
pull_varattnos((Node *) rel->reltargetlist, rel->relid, &attrs_used);
/* Add all the attributes used by restriction clauses. */
foreach(lc, rel->baserestrictinfo)
{
RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
pull_varattnos((Node *) rinfo->clause, rel->relid, &attrs_used);
}
/* Construct a bitmapset of columns stored in the index. */
for (i = 0; i < index->ncolumns; i++)
{
int attno = index->indexkeys[i];
/*
* For the moment, we just ignore index expressions. It might be nice
* to do something with them, later. We also ignore index columns
* that are system columns (such as OID), because the virtual-tuple
* coding used by IndexStoreHeapTuple() can't deal with them.
*/
if (attno <= 0)
continue;
index_attrs =
bms_add_member(index_attrs,
attno - FirstLowInvalidHeapAttributeNumber);
}
/* Do we have all the necessary attributes? */
result = bms_is_subset(attrs_used, index_attrs);
bms_free(attrs_used);
bms_free(index_attrs);
return result;
}
/****************************************************************************
* ---- ROUTINES TO CHECK RESTRICTIONS ----
****************************************************************************/