*
* orclauses.cpp
* Routines to extract restriction OR clauses from join OR clauses
*
* Portions Copyright (c) 2022 Huawei Technologies Co.,Ltd.
* Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* src/gausskernel/optimizer/util/orclauses.cpp
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "nodes/makefuncs.h"
#include "nodes/nodeFuncs.h"
#include "optimizer/clauses.h"
#include "optimizer/cost.h"
#include "optimizer/orclauses.h"
#include "optimizer/restrictinfo.h"
static bool is_safe_restriction_clause_for(RestrictInfo *rinfo, RelOptInfo *rel);
static Expr *extract_or_clause(RestrictInfo *or_rinfo, RelOptInfo *rel);
static void consider_new_or_clause(PlannerInfo *root, RelOptInfo *rel, Expr *orclause, RestrictInfo *join_or_rinfo);
* extract_restriction_or_clauses
* Examine join OR-of-AND clauses to see if any useful restriction OR
* clauses can be extracted. If so, add them to the query.
*
* Although a join clause must reference multiple relations overall,
* an OR of ANDs clause might contain sub-clauses that reference just one
* relation and can be used to build a restriction clause for that rel.
* 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 allows the latter clauses to be applied during the scans of a and b,
* perhaps as index qualifications, and in any case reducing the number of
* rows arriving at the join. 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
* would 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.) We can compensate for this
* redundancy by changing the cached selectivity of the original OR clause,
* canceling out the (valid) reduction in the estimated sizes of the base
* relations so that the estimated joinrel size remains the same. 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 doesn't work in cases where the size estimation is nonlinear
* (i.e., outer and IN joins). But it beats not doing anything.
*
* We examine each base relation to see if join clauses associated with it
* contain extractable restriction conditions. If so, add those conditions
* to the rel's baserestrictinfo and update the cached selectivities of the
* join clauses. Note that the same join clause will be examined afresh
* from the point of view of each baserel that participates in it, so its
* cached selectivity may get updated multiple times.
*/
void extract_restriction_or_clauses(PlannerInfo *root)
{
Index rti;
for (rti = 1; (int)rti < root->simple_rel_array_size; rti++) {
RelOptInfo *rel = root->simple_rel_array[rti];
ListCell *lc = NULL;
if (rel == NULL) {
continue;
}
Assert(rel->relid == rti);
if (rel->reloptkind != RELOPT_BASEREL) {
continue;
}
* Find potentially interesting OR joinclauses. We can use any
* joinclause that is considered safe to move to this rel by the
* parameterized-path machinery, even though what we are going to do
* with it is not exactly a parameterized path.
*
* However, it seems best to ignore clauses that have been marked
* redundant (by setting norm_selec > 1). That likely can't happen
* for OR clauses, but let's be safe.
*/
foreach(lc, rel->joininfo) {
RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
if (restriction_is_or_clause(rinfo) && join_clause_is_movable_to(rinfo, rel) && rinfo->norm_selec <= 1) {
Expr *orclause = extract_or_clause(rinfo, rel);
* If successful, decide whether we want to use the clause,
* and insert it into the rel's restrictinfo list if so.
*/
if (orclause) {
consider_new_or_clause(root, rel, orclause, rinfo);
}
}
}
}
}
* Is the given primitive (non-OR) RestrictInfo safe to move to the rel?
*/
static bool is_safe_restriction_clause_for(RestrictInfo *rinfo, RelOptInfo *rel)
{
* We want clauses that mention the rel, and only the rel. So in
* particular pseudoconstant clauses can be rejected quickly. Then check
* the clause's Var membership.
*/
if (rinfo->pseudoconstant) {
return false;
}
if (!bms_equal(rinfo->clause_relids, rel->relids)) {
return false;
}
if (contain_volatile_functions((Node *) rinfo->clause)) {
return false;
}
return true;
}
* Try to extract a restriction clause mentioning only "rel" from the given
* join OR-clause.
*
* We must be able to extract at least one qual for this rel from each of
* the arms of the OR, else we can't use it.
*
* Returns an OR clause (not a RestrictInfo!) pertaining to rel, or NULL
* if no OR clause could be extracted.
*/
static Expr *extract_or_clause(RestrictInfo *or_rinfo, RelOptInfo *rel)
{
List *clauselist = NIL;
ListCell *lc = NULL;
* Scan each arm of the input OR clause. Notice we descend into
* or_rinfo->orclause, which has RestrictInfo nodes embedded below the
* toplevel OR/AND structure. This is useful because we can use the info
* in those nodes to make is_safe_restriction_clause_for()'s checks
* cheaper. We'll strip those nodes from the returned tree, though,
* meaning that fresh ones will be built if the clause is accepted as a
* restriction clause. This might seem wasteful --- couldn't we re-use
* the existing RestrictInfos? But that'd require assuming that
* selectivity and other cached data is computed exactly the same way for
* a restriction clause as for a join clause, which seems undesirable.
*/
Assert(is_orclause(or_rinfo->orclause));
foreach(lc, ((BoolExpr *) or_rinfo->orclause)->args) {
Node *orarg = (Node *) lfirst(lc);
List *subclauses = NIL;
Node *subclause = NULL;
if (is_andclause(orarg)) {
List *andargs = ((BoolExpr *) orarg)->args;
ListCell *lc2 = NULL;
foreach(lc2, andargs) {
RestrictInfo *rinfo = lfirst_node(RestrictInfo, lc2);
if (restriction_is_or_clause(rinfo)) {
* Recurse to deal with nested OR. Note we *must* recurse
* here, this isn't just overly-tense optimization: we
* have to descend far enough to find and strip all
* RestrictInfos in the expression.
*/
Expr *suborclause = extract_or_clause(rinfo, rel);
if (suborclause) {
subclauses = lappend(subclauses, suborclause);
}
} else if (is_safe_restriction_clause_for(rinfo, rel)) {
subclauses = lappend(subclauses, rinfo->clause);
}
}
} else {
RestrictInfo *rinfo = castNode(RestrictInfo, orarg);
Assert(!restriction_is_or_clause(rinfo));
if (is_safe_restriction_clause_for(rinfo, rel)) {
subclauses = lappend(subclauses, rinfo->clause);
}
}
* If nothing could be extracted from this arm, we can't do anything
* with this OR clause.
*/
if (subclauses == NIL) {
return NULL;
}
* OK, add subclause(s) to the result OR. If we found more than one,
* we need an AND node. But if we found only one, and it is itself an
* OR node, add its subclauses to the result instead; this is needed
* to preserve AND/OR flatness (ie, no OR directly underneath OR).
*/
subclause = (Node *) make_ands_explicit(subclauses);
if (is_orclause(subclause)) {
clauselist = list_concat(clauselist, ((BoolExpr *) subclause)->args);
} else {
clauselist = lappend(clauselist, subclause);
}
}
* If we got a restriction clause from every arm, wrap them up in an OR
* node. (In theory the OR node might be unnecessary, if there was only
* one arm --- but then the input OR node was also redundant.)
*/
if (clauselist != NIL) {
return make_orclause(clauselist);
}
return NULL;
}
* Consider whether a successfully-extracted restriction OR clause is
* actually worth using. If so, add it to the planner's data structures,
* and adjust the original join clause (join_or_rinfo) to compensate.
*/
static void consider_new_or_clause(PlannerInfo *root, RelOptInfo *rel, Expr *orclause, RestrictInfo *join_or_rinfo)
{
RestrictInfo *or_rinfo = NULL;
Selectivity or_selec, orig_selec;
* Build a RestrictInfo from the new OR clause. We can assume it's valid
* as a base restriction clause.
*/
or_rinfo = make_restrictinfo(orclause, true, false, false, join_or_rinfo->security_level, NULL, NULL, NULL, false);
* Estimate its selectivity. (We could have done this earlier, but doing
* it on the RestrictInfo representation allows the result to get cached,
* saving work later.)
*/
or_selec = clause_selectivity(root, (Node *) or_rinfo, 0, JOIN_INNER, NULL);
* The clause is only worth adding to the query if it rejects a useful
* fraction of the base relation's rows; otherwise, it's just going to
* cause duplicate computation (since we will still have to check the
* original OR clause when the join is formed). Somewhat arbitrarily, we
* set the selectivity threshold at 0.9.
*/
if (or_selec > 0.9) {
return;
}
* OK, add it to the rel's restriction-clause list.
*/
rel->baserestrictinfo = lappend(rel->baserestrictinfo, or_rinfo);
rel->baserestrict_min_security = Min(rel->baserestrict_min_security, or_rinfo->security_level);
* Adjust the original join 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.
*
* XXX another major hack: we adjust only norm_selec, the cached
* selectivity for JOIN_INNER semantics, even though the join clause
* might've been an outer-join clause. This is partly because we can't
* easily identify the relevant SpecialJoinInfo here, and partly because
* the linearity assumption we're making would fail anyway. (If it is an
* outer-join clause, "rel" must be on the nullable side, else we'd not
* have gotten here. So the computation of the join size is going to be
* quite nonlinear with respect to the size of "rel", so it's not clear
* how we ought to adjust outer_selec even if we could compute its
* original value correctly.)
*/
if (or_selec > 0) {
SpecialJoinInfo sjinfo;
* Make up a SpecialJoinInfo for JOIN_INNER semantics. (Compare
* approx_tuple_count() in costsize.c.)
*/
sjinfo.type = T_SpecialJoinInfo;
sjinfo.min_lefthand = bms_difference(join_or_rinfo->clause_relids,
rel->relids);
sjinfo.min_righthand = rel->relids;
sjinfo.syn_lefthand = sjinfo.min_lefthand;
sjinfo.syn_righthand = sjinfo.min_righthand;
sjinfo.jointype = JOIN_INNER;
sjinfo.lhs_strict = false;
sjinfo.delay_upper_joins = false;
sjinfo.join_quals = NIL;
orig_selec = clause_selectivity(root, (Node *) join_or_rinfo, 0, JOIN_INNER, &sjinfo);
join_or_rinfo->norm_selec = orig_selec / or_selec;
if (join_or_rinfo->norm_selec > 1) {
join_or_rinfo->norm_selec = 1;
}
}
}