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Add the ability to extract OR indexscan conditions from OR-of-AND

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join conditions in which each OR subclause includes a constraint on
the same relation.  This implements the other useful side-effect of
conversion to CNF format, without its unpleasant side-effects.  As
per pghackers discussion of a few weeks ago.
This commit is contained in:
Tom Lane
2004-01-05 05:07:36 +00:00
parent bf488a6842
commit 9091e8d1b2
16 changed files with 438 additions and 295 deletions
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211
src/backend/optimizer/path/orindxpath.c
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@ -8,20 +8,21 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/orindxpath.c,v 1.55 2004/01/04 00:07:32 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/orindxpath.c,v 1.56 2004/01/05 05:07:35 tgl Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "optimizer/clauses.h"
#include "optimizer/cost.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/restrictinfo.h"
static IndexPath *best_or_subclause_indices(Query *root, RelOptInfo *rel,
static IndexPath *best_or_subclause_indexes(Query *root, RelOptInfo *rel,
List *subclauses);
static bool best_or_subclause_index(Query *root,
RelOptInfo *rel,
@ -32,16 +33,166 @@ static bool best_or_subclause_index(Query *root,
Cost *retTotalCost);
/*----------
* create_or_index_quals
* Examine join OR-of-AND quals to see if any useful restriction OR
* clauses can be extracted. If so, add them to the query.
*
* Although a join clause must reference other relations overall,
* an OR of ANDs clause might contain sub-clauses that reference just this
* relation and can be used to build a restriction clause.
* For example consider
* WHERE ((a.x = 42 AND b.y = 43) OR (a.x = 44 AND b.z = 45));
* We can transform this into
* WHERE ((a.x = 42 AND b.y = 43) OR (a.x = 44 AND b.z = 45))
* AND (a.x = 42 OR a.x = 44)
* AND (b.y = 43 OR b.z = 45);
* which opens the potential to build OR indexscans on a and b. In essence
* this is a partial transformation to CNF (AND of ORs format). It is not
* complete, however, because we do not unravel the original OR --- doing so
* would usually bloat the qualification expression to little gain.
*
* The added quals are partially redundant with the original OR, and therefore
* will cause the size of the joinrel to be underestimated when it is finally
* formed. (This would be true of a full transformation to CNF as well; the
* fault is not really in the transformation, but in clauselist_selectivity's
* inability to recognize redundant conditions.) To minimize the collateral
* damage, we want to minimize the number of quals added. Therefore we do
* not add every possible extracted restriction condition to the query.
* Instead, we search for the single restriction condition that generates
* the most useful (cheapest) OR indexscan, and add only that condition.
* This is a pretty ad-hoc heuristic, but quite useful.
*
* We can then compensate for the redundancy of the added qual by poking
* the recorded selectivity of the original OR clause, thereby ensuring
* the added qual doesn't change the estimated size of the joinrel when
* it is finally formed. This is a MAJOR HACK: it depends on the fact
* that clause selectivities are cached and on the fact that the same
* RestrictInfo node will appear in every joininfo list that might be used
* when the joinrel is formed. And it probably isn't right in cases where
* the size estimation is nonlinear (i.e., outer and IN joins). But it
* beats not doing anything.
*
* NOTE: one might think this messiness could be worked around by generating
* the indexscan path with a small path->rows value, and not touching the
* rel's baserestrictinfo or rel->rows. However, that does not work.
* The optimizer's fundamental design assumes that every general-purpose
* Path for a given relation generates the same number of rows. Without
* this assumption we'd not be able to optimize solely on the cost of Paths,
* but would have to take number of output rows into account as well.
* (Perhaps someday that'd be worth doing, but it's a pretty big change...)
*
* 'rel' is the relation entry for which quals are to be created
*
* If successful, adds qual(s) to rel->baserestrictinfo and returns TRUE.
* If no quals available, returns FALSE and doesn't change rel.
*
* Note: check_partial_indexes() must have been run previously.
*----------
*/
bool
create_or_index_quals(Query *root, RelOptInfo *rel)
{
IndexPath *bestpath = NULL;
RestrictInfo *bestrinfo = NULL;
FastList orclauses;
List *orclause;
Expr *indxqual_or_expr;
RestrictInfo *or_rinfo;
Selectivity or_selec,
orig_selec;
List *i;
/*
* We use the best_or_subclause_indexes() machinery to locate the
* best combination of restriction subclauses. Note we must ignore
* any joinclauses that are not marked valid_everywhere, because they
* cannot be pushed down due to outer-join rules.
*/
foreach(i, rel->joininfo)
{
JoinInfo *joininfo = (JoinInfo *) lfirst(i);
List *j;
foreach(j, joininfo->jinfo_restrictinfo)
{
RestrictInfo *rinfo = (RestrictInfo *) lfirst(j);
if (restriction_is_or_clause(rinfo) &&
rinfo->valid_everywhere)
{
IndexPath *pathnode;
pathnode = best_or_subclause_indexes(root,
rel,
((BoolExpr *) rinfo->orclause)->args);
if (pathnode)
{
if (bestpath == NULL ||
pathnode->path.total_cost < bestpath->path.total_cost)
{
bestpath = pathnode;
bestrinfo = rinfo;
}
}
}
}
}
/* Fail if no suitable clauses found */
if (bestpath == NULL)
return false;
/*
* Build an expression representation of the indexqual, expanding
* the implicit OR and AND semantics of the first- and
* second-level lists.
*/
FastListInit(&orclauses);
foreach(orclause, bestpath->indexqual)
FastAppend(&orclauses, make_ands_explicit(lfirst(orclause)));
indxqual_or_expr = make_orclause(FastListValue(&orclauses));
/*
* And add it to the rel's restriction list.
*/
or_rinfo = make_restrictinfo(indxqual_or_expr, true, true);
rel->baserestrictinfo = lappend(rel->baserestrictinfo, or_rinfo);
/*
* Adjust the original OR clause's cached selectivity to compensate
* for the selectivity of the added (but redundant) lower-level qual.
* This should result in the join rel getting approximately the same
* rows estimate as it would have gotten without all these shenanigans.
* (XXX major hack alert ... this depends on the assumption that the
* selectivity will stay cached ...)
*/
or_selec = clause_selectivity(root, (Node *) or_rinfo,
0, JOIN_INNER);
if (or_selec > 0 && or_selec < 1)
{
orig_selec = clause_selectivity(root, (Node *) bestrinfo,
0, JOIN_INNER);
bestrinfo->this_selec = orig_selec / or_selec;
/* clamp result to sane range */
if (bestrinfo->this_selec > 1)
bestrinfo->this_selec = 1;
}
/* Tell caller to recompute rel's rows estimate */
return true;
}
/*
* create_or_index_paths
* Creates multi-scan index paths for indices that match OR clauses.
* Creates multi-scan index paths for indexes that match OR clauses.
*
* 'rel' is the relation entry for which the paths are to be created
*
* Returns nothing, but adds paths to rel->pathlist via add_path().
*
* Note: create_index_paths() must have been run already, since it does
* the heavy lifting to determine whether partial indexes may be used.
* Note: check_partial_indexes() must have been run previously.
*/
void
create_or_index_paths(Query *root, RelOptInfo *rel)
@ -60,7 +211,7 @@ create_or_index_paths(Query *root, RelOptInfo *rel)
{
IndexPath *pathnode;
pathnode = best_or_subclause_indices(root,
pathnode = best_or_subclause_indexes(root,
rel,
((BoolExpr *) rinfo->orclause)->args);
@ -68,49 +219,10 @@ create_or_index_paths(Query *root, RelOptInfo *rel)
add_path(rel, (Path *) pathnode);
}
}
/*
* Also consider join clauses that are ORs. Although a join clause
* must reference other relations overall, an OR of ANDs clause might
* contain sub-clauses that reference just our relation and can be
* used to build a non-join indexscan. For example consider
* WHERE (a.x = 42 AND b.y = 43) OR (a.x = 44 AND b.z = 45);
* We could build an OR indexscan on a.x using those subclauses.
*
* XXX don't enable this code quite yet. Although the plans it creates
* are correct, and possibly even useful, we are totally confused about
* the number of rows returned, leading to poor choices of join plans
* above the indexscan. Need to restructure the way join sizes are
* calculated before this will really work.
*/
#ifdef NOT_YET
foreach(i, rel->joininfo)
{
JoinInfo *joininfo = (JoinInfo *) lfirst(i);
List *j;
foreach(j, joininfo->jinfo_restrictinfo)
{
RestrictInfo *rinfo = (RestrictInfo *) lfirst(j);
if (restriction_is_or_clause(rinfo))
{
IndexPath *pathnode;
pathnode = best_or_subclause_indices(root,
rel,
((BoolExpr *) rinfo->orclause)->args);
if (pathnode)
add_path(rel, (Path *) pathnode);
}
}
}
#endif
}
/*
* best_or_subclause_indices
* best_or_subclause_indexes
* Determine the best index to be used in conjunction with each subclause
* of an OR clause, and build a Path for a multi-index scan.
*
@ -134,7 +246,7 @@ create_or_index_paths(Query *root, RelOptInfo *rel)
* single tuple more than once).
*/
static IndexPath *
best_or_subclause_indices(Query *root,
best_or_subclause_indexes(Query *root,
RelOptInfo *rel,
List *subclauses)
{
@ -202,7 +314,10 @@ best_or_subclause_indices(Query *root,
/* We don't actually care what order the index scans in. */
pathnode->indexscandir = NoMovementScanDirection;
/* XXX this may be wrong when using join OR clauses... */
/*
* The number of rows is the same as the parent rel's estimate, since
* this isn't a join inner indexscan.
*/
pathnode->rows = rel->rows;
return pathnode;