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orclauses.c
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1 /*-------------------------------------------------------------------------
2  *
3  * orclauses.c
4  * Routines to extract restriction OR clauses from join OR clauses
5  *
6  * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
7  * Portions Copyright (c) 1994, Regents of the University of California
8  *
9  *
10  * IDENTIFICATION
11  * src/backend/optimizer/util/orclauses.c
12  *
13  *-------------------------------------------------------------------------
14  */
15 
16 #include "postgres.h"
17 
18 #include "nodes/makefuncs.h"
19 #include "nodes/nodeFuncs.h"
20 #include "optimizer/optimizer.h"
21 #include "optimizer/orclauses.h"
22 #include "optimizer/paths.h"
23 #include "optimizer/restrictinfo.h"
24 
25 
27 static Expr *extract_or_clause(RestrictInfo *or_rinfo, RelOptInfo *rel);
29  Expr *orclause, RestrictInfo *join_or_rinfo);
30 
31 
32 /*
33  * extract_restriction_or_clauses
34  * Examine join OR-of-AND clauses to see if any useful restriction OR
35  * clauses can be extracted. If so, add them to the query.
36  *
37  * Although a join clause must reference multiple relations overall,
38  * an OR of ANDs clause might contain sub-clauses that reference just one
39  * relation and can be used to build a restriction clause for that rel.
40  * For example consider
41  * WHERE ((a.x = 42 AND b.y = 43) OR (a.x = 44 AND b.z = 45));
42  * We can transform this into
43  * WHERE ((a.x = 42 AND b.y = 43) OR (a.x = 44 AND b.z = 45))
44  * AND (a.x = 42 OR a.x = 44)
45  * AND (b.y = 43 OR b.z = 45);
46  * which allows the latter clauses to be applied during the scans of a and b,
47  * perhaps as index qualifications, and in any case reducing the number of
48  * rows arriving at the join. In essence this is a partial transformation to
49  * CNF (AND of ORs format). It is not complete, however, because we do not
50  * unravel the original OR --- doing so would usually bloat the qualification
51  * expression to little gain.
52  *
53  * The added quals are partially redundant with the original OR, and therefore
54  * would cause the size of the joinrel to be underestimated when it is finally
55  * formed. (This would be true of a full transformation to CNF as well; the
56  * fault is not really in the transformation, but in clauselist_selectivity's
57  * inability to recognize redundant conditions.) We can compensate for this
58  * redundancy by changing the cached selectivity of the original OR clause,
59  * canceling out the (valid) reduction in the estimated sizes of the base
60  * relations so that the estimated joinrel size remains the same. This is
61  * a MAJOR HACK: it depends on the fact that clause selectivities are cached
62  * and on the fact that the same RestrictInfo node will appear in every
63  * joininfo list that might be used when the joinrel is formed.
64  * And it doesn't work in cases where the size estimation is nonlinear
65  * (i.e., outer and IN joins). But it beats not doing anything.
66  *
67  * We examine each base relation to see if join clauses associated with it
68  * contain extractable restriction conditions. If so, add those conditions
69  * to the rel's baserestrictinfo and update the cached selectivities of the
70  * join clauses. Note that the same join clause will be examined afresh
71  * from the point of view of each baserel that participates in it, so its
72  * cached selectivity may get updated multiple times.
73  */
74 void
76 {
77  Index rti;
78 
79  /* Examine each baserel for potential join OR clauses */
80  for (rti = 1; rti < root->simple_rel_array_size; rti++)
81  {
82  RelOptInfo *rel = root->simple_rel_array[rti];
83  ListCell *lc;
84 
85  /* there may be empty slots corresponding to non-baserel RTEs */
86  if (rel == NULL)
87  continue;
88 
89  Assert(rel->relid == rti); /* sanity check on array */
90 
91  /* ignore RTEs that are "other rels" */
92  if (rel->reloptkind != RELOPT_BASEREL)
93  continue;
94 
95  /*
96  * Find potentially interesting OR joinclauses. We can use any
97  * joinclause that is considered safe to move to this rel by the
98  * parameterized-path machinery, even though what we are going to do
99  * with it is not exactly a parameterized path.
100  */
101  foreach(lc, rel->joininfo)
102  {
103  RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
104 
105  if (restriction_is_or_clause(rinfo) &&
106  join_clause_is_movable_to(rinfo, rel))
107  {
108  /* Try to extract a qual for this rel only */
109  Expr *orclause = extract_or_clause(rinfo, rel);
110 
111  /*
112  * If successful, decide whether we want to use the clause,
113  * and insert it into the rel's restrictinfo list if so.
114  */
115  if (orclause)
116  consider_new_or_clause(root, rel, orclause, rinfo);
117  }
118  }
119  }
120 }
121 
122 /*
123  * Is the given primitive (non-OR) RestrictInfo safe to move to the rel?
124  */
125 static bool
127 {
128  /*
129  * We want clauses that mention the rel, and only the rel. So in
130  * particular pseudoconstant clauses can be rejected quickly. Then check
131  * the clause's Var membership.
132  */
133  if (rinfo->pseudoconstant)
134  return false;
135  if (!bms_equal(rinfo->clause_relids, rel->relids))
136  return false;
137 
138  /* We don't want extra evaluations of any volatile functions */
139  if (contain_volatile_functions((Node *) rinfo->clause))
140  return false;
141 
142  return true;
143 }
144 
145 /*
146  * Try to extract a restriction clause mentioning only "rel" from the given
147  * join OR-clause.
148  *
149  * We must be able to extract at least one qual for this rel from each of
150  * the arms of the OR, else we can't use it.
151  *
152  * Returns an OR clause (not a RestrictInfo!) pertaining to rel, or NULL
153  * if no OR clause could be extracted.
154  */
155 static Expr *
157 {
158  List *clauselist = NIL;
159  ListCell *lc;
160 
161  /*
162  * Scan each arm of the input OR clause. Notice we descend into
163  * or_rinfo->orclause, which has RestrictInfo nodes embedded below the
164  * toplevel OR/AND structure. This is useful because we can use the info
165  * in those nodes to make is_safe_restriction_clause_for()'s checks
166  * cheaper. We'll strip those nodes from the returned tree, though,
167  * meaning that fresh ones will be built if the clause is accepted as a
168  * restriction clause. This might seem wasteful --- couldn't we re-use
169  * the existing RestrictInfos? But that'd require assuming that
170  * selectivity and other cached data is computed exactly the same way for
171  * a restriction clause as for a join clause, which seems undesirable.
172  */
173  Assert(is_orclause(or_rinfo->orclause));
174  foreach(lc, ((BoolExpr *) or_rinfo->orclause)->args)
175  {
176  Node *orarg = (Node *) lfirst(lc);
177  List *subclauses = NIL;
178  Node *subclause;
179 
180  /* OR arguments should be ANDs or sub-RestrictInfos */
181  if (is_andclause(orarg))
182  {
183  List *andargs = ((BoolExpr *) orarg)->args;
184  ListCell *lc2;
185 
186  foreach(lc2, andargs)
187  {
188  RestrictInfo *rinfo = lfirst_node(RestrictInfo, lc2);
189 
190  if (restriction_is_or_clause(rinfo))
191  {
192  /*
193  * Recurse to deal with nested OR. Note we *must* recurse
194  * here, this isn't just overly-tense optimization: we
195  * have to descend far enough to find and strip all
196  * RestrictInfos in the expression.
197  */
198  Expr *suborclause;
199 
200  suborclause = extract_or_clause(rinfo, rel);
201  if (suborclause)
202  subclauses = lappend(subclauses, suborclause);
203  }
204  else if (is_safe_restriction_clause_for(rinfo, rel))
205  subclauses = lappend(subclauses, rinfo->clause);
206  }
207  }
208  else
209  {
210  RestrictInfo *rinfo = castNode(RestrictInfo, orarg);
211 
213  if (is_safe_restriction_clause_for(rinfo, rel))
214  subclauses = lappend(subclauses, rinfo->clause);
215  }
216 
217  /*
218  * If nothing could be extracted from this arm, we can't do anything
219  * with this OR clause.
220  */
221  if (subclauses == NIL)
222  return NULL;
223 
224  /*
225  * OK, add subclause(s) to the result OR. If we found more than one,
226  * we need an AND node. But if we found only one, and it is itself an
227  * OR node, add its subclauses to the result instead; this is needed
228  * to preserve AND/OR flatness (ie, no OR directly underneath OR).
229  */
230  subclause = (Node *) make_ands_explicit(subclauses);
231  if (is_orclause(subclause))
232  clauselist = list_concat(clauselist,
233  ((BoolExpr *) subclause)->args);
234  else
235  clauselist = lappend(clauselist, subclause);
236  }
237 
238  /*
239  * If we got a restriction clause from every arm, wrap them up in an OR
240  * node. (In theory the OR node might be unnecessary, if there was only
241  * one arm --- but then the input OR node was also redundant.)
242  */
243  if (clauselist != NIL)
244  return make_orclause(clauselist);
245  return NULL;
246 }
247 
248 /*
249  * Consider whether a successfully-extracted restriction OR clause is
250  * actually worth using. If so, add it to the planner's data structures,
251  * and adjust the original join clause (join_or_rinfo) to compensate.
252  */
253 static void
255  Expr *orclause, RestrictInfo *join_or_rinfo)
256 {
257  RestrictInfo *or_rinfo;
258  Selectivity or_selec,
259  orig_selec;
260 
261  /*
262  * Build a RestrictInfo from the new OR clause. We can assume it's valid
263  * as a base restriction clause.
264  */
265  or_rinfo = make_restrictinfo(root,
266  orclause,
267  true,
268  false,
269  false,
270  false,
271  join_or_rinfo->security_level,
272  NULL,
273  NULL,
274  NULL);
275 
276  /*
277  * Estimate its selectivity. (We could have done this earlier, but doing
278  * it on the RestrictInfo representation allows the result to get cached,
279  * saving work later.)
280  */
281  or_selec = clause_selectivity(root, (Node *) or_rinfo,
282  0, JOIN_INNER, NULL);
283 
284  /*
285  * The clause is only worth adding to the query if it rejects a useful
286  * fraction of the base relation's rows; otherwise, it's just going to
287  * cause duplicate computation (since we will still have to check the
288  * original OR clause when the join is formed). Somewhat arbitrarily, we
289  * set the selectivity threshold at 0.9.
290  */
291  if (or_selec > 0.9)
292  return; /* forget it */
293 
294  /*
295  * OK, add it to the rel's restriction-clause list.
296  */
297  rel->baserestrictinfo = lappend(rel->baserestrictinfo, or_rinfo);
299  or_rinfo->security_level);
300 
301  /*
302  * Adjust the original join OR clause's cached selectivity to compensate
303  * for the selectivity of the added (but redundant) lower-level qual. This
304  * should result in the join rel getting approximately the same rows
305  * estimate as it would have gotten without all these shenanigans.
306  *
307  * XXX major hack alert: this depends on the assumption that the
308  * selectivity will stay cached.
309  *
310  * XXX another major hack: we adjust only norm_selec, the cached
311  * selectivity for JOIN_INNER semantics, even though the join clause
312  * might've been an outer-join clause. This is partly because we can't
313  * easily identify the relevant SpecialJoinInfo here, and partly because
314  * the linearity assumption we're making would fail anyway. (If it is an
315  * outer-join clause, "rel" must be on the nullable side, else we'd not
316  * have gotten here. So the computation of the join size is going to be
317  * quite nonlinear with respect to the size of "rel", so it's not clear
318  * how we ought to adjust outer_selec even if we could compute its
319  * original value correctly.)
320  */
321  if (or_selec > 0)
322  {
323  SpecialJoinInfo sjinfo;
324 
325  /*
326  * Make up a SpecialJoinInfo for JOIN_INNER semantics. (Compare
327  * approx_tuple_count() in costsize.c.)
328  */
329  init_dummy_sjinfo(&sjinfo,
330  bms_difference(join_or_rinfo->clause_relids,
331  rel->relids),
332  rel->relids);
333 
334  /* Compute inner-join size */
335  orig_selec = clause_selectivity(root, (Node *) join_or_rinfo,
336  0, JOIN_INNER, &sjinfo);
337 
338  /* And hack cached selectivity so join size remains the same */
339  join_or_rinfo->norm_selec = orig_selec / or_selec;
340  /* ensure result stays in sane range */
341  if (join_or_rinfo->norm_selec > 1)
342  join_or_rinfo->norm_selec = 1;
343  /* as explained above, we don't touch outer_selec */
344  }
345 }
bool bms_equal(const Bitmapset *a, const Bitmapset *b)
Definition: bitmapset.c:142
Bitmapset * bms_difference(const Bitmapset *a, const Bitmapset *b)
Definition: bitmapset.c:346
#define Min(x, y)
Definition: c.h:1004
#define Assert(condition)
Definition: c.h:858
unsigned int Index
Definition: c.h:614
bool contain_volatile_functions(Node *clause)
Definition: clauses.c:538
Selectivity clause_selectivity(PlannerInfo *root, Node *clause, int varRelid, JoinType jointype, SpecialJoinInfo *sjinfo)
Definition: clausesel.c:667
void init_dummy_sjinfo(SpecialJoinInfo *sjinfo, Relids left_relids, Relids right_relids)
Definition: joinrels.c:669
List * lappend(List *list, void *datum)
Definition: list.c:339
List * list_concat(List *list1, const List *list2)
Definition: list.c:561
Expr * make_ands_explicit(List *andclauses)
Definition: makefuncs.c:726
Expr * make_orclause(List *orclauses)
Definition: makefuncs.c:670
static bool is_andclause(const void *clause)
Definition: nodeFuncs.h:105
static bool is_orclause(const void *clause)
Definition: nodeFuncs.h:114
double Selectivity
Definition: nodes.h:250
#define castNode(_type_, nodeptr)
Definition: nodes.h:176
@ JOIN_INNER
Definition: nodes.h:293
void extract_restriction_or_clauses(PlannerInfo *root)
Definition: orclauses.c:75
static void consider_new_or_clause(PlannerInfo *root, RelOptInfo *rel, Expr *orclause, RestrictInfo *join_or_rinfo)
Definition: orclauses.c:254
static Expr * extract_or_clause(RestrictInfo *or_rinfo, RelOptInfo *rel)
Definition: orclauses.c:156
static bool is_safe_restriction_clause_for(RestrictInfo *rinfo, RelOptInfo *rel)
Definition: orclauses.c:126
@ RELOPT_BASEREL
Definition: pathnodes.h:817
#define lfirst(lc)
Definition: pg_list.h:172
#define lfirst_node(type, lc)
Definition: pg_list.h:176
#define NIL
Definition: pg_list.h:68
tree ctl root
Definition: radixtree.h:1880
bool restriction_is_or_clause(RestrictInfo *restrictinfo)
Definition: restrictinfo.c:416
RestrictInfo * make_restrictinfo(PlannerInfo *root, Expr *clause, bool is_pushed_down, bool has_clone, bool is_clone, bool pseudoconstant, Index security_level, Relids required_relids, Relids incompatible_relids, Relids outer_relids)
Definition: restrictinfo.c:63
bool join_clause_is_movable_to(RestrictInfo *rinfo, RelOptInfo *baserel)
Definition: restrictinfo.c:584
Definition: pg_list.h:54
Definition: nodes.h:129
List * baserestrictinfo
Definition: pathnodes.h:975
List * joininfo
Definition: pathnodes.h:981
Relids relids
Definition: pathnodes.h:861
Index relid
Definition: pathnodes.h:908
RelOptKind reloptkind
Definition: pathnodes.h:855
Index baserestrict_min_security
Definition: pathnodes.h:979
Index security_level
Definition: pathnodes.h:2574
Expr * clause
Definition: pathnodes.h:2552