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/*-------------------------------------------------------------------------
*
* orindxpath.c
* Routines to find index paths that match a set of OR clauses
*
* Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/orindxpath.c,v 1.90 2009/06/11 14:48:59 momjian Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "optimizer/cost.h"
#include "optimizer/paths.h"
#include "optimizer/restrictinfo.h"
/*----------
* 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(PlannerInfo *root, RelOptInfo *rel)
{
BitmapOrPath *bestpath = NULL;
RestrictInfo *bestrinfo = NULL;
List *newrinfos;
RestrictInfo *or_rinfo;
Selectivity or_selec,
orig_selec;
ListCell *i;
/*
* Find potentially interesting OR joinclauses.
*
* We must ignore clauses for which the target rel is in nullable_relids;
* that means there's an outer join below the clause and so it can't be
* enforced at the relation scan level.
*
* We must also ignore clauses that are marked !is_pushed_down (ie they
* are themselves outer-join clauses). It would be safe to extract an
* index condition from such a clause if we are within the nullable rather
* than the non-nullable side of its join, but we haven't got enough
* context here to tell which applies. OR clauses in outer-join quals
* aren't exactly common, so we'll let that case go unoptimized for now.
*/
foreach(i, rel->joininfo)
{
RestrictInfo *rinfo = (RestrictInfo *) lfirst(i);
if (restriction_is_or_clause(rinfo) &&
rinfo->is_pushed_down &&
!bms_is_member(rel->relid, rinfo->nullable_relids))
{
/*
* Use the generate_bitmap_or_paths() machinery to estimate the
* value of each OR clause. We can use regular restriction
* clauses along with the OR clause contents to generate
* indexquals. We pass outer_rel = NULL so that sub-clauses that
* are actually joins will be ignored.
*/
List *orpaths;
ListCell *k;
orpaths = generate_bitmap_or_paths(root, rel,
list_make1(rinfo),
rel->baserestrictinfo,
NULL);
/* Locate the cheapest OR path */
foreach(k, orpaths)
{
BitmapOrPath *path = (BitmapOrPath *) lfirst(k);
Assert(IsA(path, BitmapOrPath));
if (bestpath == NULL ||
path->path.total_cost < bestpath->path.total_cost)
{
bestpath = path;
bestrinfo = rinfo;
}
}
}
}
/* Fail if no suitable clauses found */
if (bestpath == NULL)
return false;
/*
* Convert the path's indexclauses structure to a RestrictInfo tree. We
* include any partial-index predicates so as to get a reasonable
* representation of what the path is actually scanning.
*/
newrinfos = make_restrictinfo_from_bitmapqual((Path *) bestpath,
true, true);
/* It's possible we get back something other than a single OR clause */
if (list_length(newrinfos) != 1)
return false;
or_rinfo = (RestrictInfo *) linitial(newrinfos);
Assert(IsA(or_rinfo, RestrictInfo));
if (!restriction_is_or_clause(or_rinfo))
return false;
/*
* OK, add it to the rel's restriction list.
*/
rel->baserestrictinfo = list_concat(rel->baserestrictinfo, newrinfos);
/*
* 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, NULL);
if (or_selec > 0 && or_selec < 1)
{
orig_selec = clause_selectivity(root, (Node *) bestrinfo,
0, JOIN_INNER, NULL);
bestrinfo->norm_selec = orig_selec / or_selec;
/* clamp result to sane range */
if (bestrinfo->norm_selec > 1)
bestrinfo->norm_selec = 1;
/* It isn't an outer join clause, so no need to adjust outer_selec */
}
/* Tell caller to recompute partial index status and rowcount estimate */
return true;
}
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