1
/*-------------------------------------------------------------------------
4
* Routines to plan a single query
6
* What's in a name, anyway? The top-level entry point of the planner/
7
* optimizer is over in planner.c, not here as you might think from the
8
* file name. But this is the main code for planning a basic join operation,
9
* shorn of features like subselects, inheritance, aggregates, grouping,
10
* and so on. (Those are the things planner.c deals with.)
12
* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
13
* Portions Copyright (c) 1994, Regents of the University of California
17
* $PostgreSQL: pgsql/src/backend/optimizer/plan/planmain.c,v 1.81 2004-12-31 22:00:09 pgsql Exp $
19
*-------------------------------------------------------------------------
23
#include "optimizer/clauses.h"
24
#include "optimizer/cost.h"
25
#include "optimizer/pathnode.h"
26
#include "optimizer/paths.h"
27
#include "optimizer/planmain.h"
30
/*--------------------
32
* Generate a path (that is, a simplified plan) for a basic query,
33
* which may involve joins but not any fancier features.
35
* Since query_planner does not handle the toplevel processing (grouping,
36
* sorting, etc) it cannot select the best path by itself. It selects
37
* two paths: the cheapest path that produces all the required tuples,
38
* independent of any ordering considerations, and the cheapest path that
39
* produces the expected fraction of the required tuples in the required
40
* ordering, if there is a path that is cheaper for this than just sorting
41
* the output of the cheapest overall path. The caller (grouping_planner)
42
* will make the final decision about which to use.
45
* root is the query to plan
46
* tlist is the target list the query should produce (NOT root->targetList!)
47
* tuple_fraction is the fraction of tuples we expect will be retrieved
50
* *cheapest_path receives the overall-cheapest path for the query
51
* *sorted_path receives the cheapest presorted path for the query,
52
* if any (NULL if there is no useful presorted path)
54
* Note: the Query node also includes a query_pathkeys field, which is both
55
* an input and an output of query_planner(). The input value signals
56
* query_planner that the indicated sort order is wanted in the final output
57
* plan. But this value has not yet been "canonicalized", since the needed
58
* info does not get computed until we scan the qual clauses. We canonicalize
59
* it as soon as that task is done. (The main reason query_pathkeys is a
60
* Query field and not a passed parameter is that the low-level routines in
61
* indxpath.c need to see it.)
63
* tuple_fraction is interpreted as follows:
64
* 0: expect all tuples to be retrieved (normal case)
65
* 0 < tuple_fraction < 1: expect the given fraction of tuples available
66
* from the plan to be retrieved
67
* tuple_fraction >= 1: tuple_fraction is the absolute number of tuples
68
* expected to be retrieved (ie, a LIMIT specification)
72
query_planner(Query *root, List *tlist, double tuple_fraction,
73
Path **cheapest_path, Path **sorted_path)
76
RelOptInfo *final_rel;
81
* If the query has an empty join tree, then it's something easy like
82
* "SELECT 2+2;" or "INSERT ... VALUES()". Fall through quickly.
84
if (root->jointree->fromlist == NIL)
86
*cheapest_path = (Path *) create_result_path(NULL, NULL,
87
(List *) root->jointree->quals);
93
* Pull out any non-variable WHERE clauses so these can be put in a
94
* toplevel "Result" node, where they will gate execution of the whole
95
* plan (the Result will not invoke its descendant plan unless the
96
* quals are true). Note that any *really* non-variable quals will
97
* have been optimized away by eval_const_expressions(). What we're
98
* mostly interested in here is quals that depend only on outer-level
99
* vars, although if the qual reduces to "WHERE FALSE" this path will
102
root->jointree->quals = (Node *)
103
pull_constant_clauses((List *) root->jointree->quals,
107
* init planner lists to empty
109
* NOTE: in_info_list was set up by subquery_planner, do not touch here
111
root->base_rel_list = NIL;
112
root->other_rel_list = NIL;
113
root->join_rel_list = NIL;
114
root->equi_key_list = NIL;
117
* Construct RelOptInfo nodes for all base relations in query.
119
add_base_rels_to_query(root, (Node *) root->jointree);
122
* Examine the targetlist and qualifications, adding entries to
123
* baserel targetlists for all referenced Vars. Restrict and join
124
* clauses are added to appropriate lists belonging to the mentioned
125
* relations. We also build lists of equijoined keys for pathkey
128
* Note: all subplan nodes will have "flat" (var-only) tlists. This
129
* implies that all expression evaluations are done at the root of the
130
* plan tree. Once upon a time there was code to try to push
131
* expensive function calls down to lower plan nodes, but that's dead
132
* code and has been for a long time...
134
build_base_rel_tlists(root, tlist);
136
(void) distribute_quals_to_rels(root, (Node *) root->jointree);
139
* Use the completed lists of equijoined keys to deduce any implied
140
* but unstated equalities (for example, A=B and B=C imply A=C).
142
generate_implied_equalities(root);
145
* We should now have all the pathkey equivalence sets built, so it's
146
* now possible to convert the requested query_pathkeys to canonical
149
root->query_pathkeys = canonicalize_pathkeys(root, root->query_pathkeys);
152
* Ready to do the primary planning.
154
final_rel = make_one_rel(root);
156
if (!final_rel || !final_rel->cheapest_total_path)
157
elog(ERROR, "failed to construct the join relation");
160
* Now that we have an estimate of the final rel's size, we can
161
* convert a tuple_fraction specified as an absolute count (ie, a
162
* LIMIT option) into a fraction of the total tuples.
164
if (tuple_fraction >= 1.0)
165
tuple_fraction /= final_rel->rows;
168
* Pick out the cheapest-total path and the cheapest presorted path
169
* for the requested pathkeys (if there is one). We should take the
170
* tuple fraction into account when selecting the cheapest presorted
171
* path, but not when selecting the cheapest-total path, since if we
172
* have to sort then we'll have to fetch all the tuples. (But there's
173
* a special case: if query_pathkeys is NIL, meaning order doesn't
174
* matter, then the "cheapest presorted" path will be the cheapest
175
* overall for the tuple fraction.)
177
* The cheapest-total path is also the one to use if grouping_planner
178
* decides to use hashed aggregation, so we return it separately even
179
* if this routine thinks the presorted path is the winner.
181
cheapestpath = final_rel->cheapest_total_path;
184
get_cheapest_fractional_path_for_pathkeys(final_rel->pathlist,
185
root->query_pathkeys,
188
/* Don't return same path in both guises; just wastes effort */
189
if (sortedpath == cheapestpath)
193
* Forget about the presorted path if it would be cheaper to sort the
194
* cheapest-total path. Here we need consider only the behavior at
195
* the tuple fraction point.
199
Path sort_path; /* dummy for result of cost_sort */
201
if (root->query_pathkeys == NIL ||
202
pathkeys_contained_in(root->query_pathkeys,
203
cheapestpath->pathkeys))
205
/* No sort needed for cheapest path */
206
sort_path.startup_cost = cheapestpath->startup_cost;
207
sort_path.total_cost = cheapestpath->total_cost;
211
/* Figure cost for sorting */
212
cost_sort(&sort_path, root, root->query_pathkeys,
213
cheapestpath->total_cost,
214
final_rel->rows, final_rel->width);
217
if (compare_fractional_path_costs(sortedpath, &sort_path,
220
/* Presorted path is a loser */
226
* If we have constant quals, add a toplevel Result step to process
231
cheapestpath = (Path *) create_result_path(final_rel,
235
sortedpath = (Path *) create_result_path(final_rel,
240
*cheapest_path = cheapestpath;
241
*sorted_path = sortedpath;