1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
|
/* Copyright (c) 2020, 2025, Oracle and/or its affiliates.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License, version 2.0,
as published by the Free Software Foundation.
This program is designed to work with certain software (including
but not limited to OpenSSL) that is licensed under separate terms,
as designated in a particular file or component or in included license
documentation. The authors of MySQL hereby grant you an additional
permission to link the program and your derivative works with the
separately licensed software that they have either included with
the program or referenced in the documentation.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License, version 2.0, for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
#ifndef SQL_JOIN_OPTIMIZER_COST_MODEL_H_
#define SQL_JOIN_OPTIMIZER_COST_MODEL_H_
#include "my_base.h"
#include "sql/join_optimizer/access_path.h"
#include "sql/join_optimizer/relational_expression.h"
#include "sql/mem_root_array.h"
struct AccessPath;
struct ContainedSubquery;
class Item;
class Query_block;
class THD;
struct TABLE;
/**
When we make cost estimates, we use this as the maximal length the
values we get from evaluating an Item (in bytes). Actual values of
e.g. blobs may be much longer, but even so we use this as an upper
limit when doing cost calculations. (For context, @see Item#max_length .)
*/
constexpr size_t kMaxItemLengthEstimate = 4096;
// These are extremely arbitrary cost model constants. We should revise them
// based on actual query times (possibly using linear regression?), and then
// put them into the cost model to make them user-tunable.
constexpr double kApplyOneFilterCost = 0.1;
constexpr double kAggregateOneRowCost = 0.1;
constexpr double kSortOneRowCost = 0.1;
constexpr double kHashBuildOneRowCost = 0.1;
constexpr double kHashProbeOneRowCost = 0.1;
constexpr double kHashReturnOneRowCost = 0.07;
constexpr double kMaterializeOneRowCost = 0.1;
constexpr double kWindowOneRowCost = 0.1;
/// A fallback cardinality estimate that is used in case the storage engine
/// cannot provide one (like for table functions). It's a fairly arbitrary
/// non-zero value.
constexpr ha_rows kRowEstimateFallback = 1000;
/// See EstimateFilterCost.
struct FilterCost {
/// Cost of evaluating the filter for all rows if subqueries are not
/// materialized. (Note that this includes the contribution from
/// init_cost_if_not_materialized.)
double cost_if_not_materialized{0.0};
/// Initial cost before the filter can be applied for the first time.
/// Typically the cost of executing 'independent subquery' in queries like:
/// "SELECT * FROM tab WHERE field = <independent subquery>".
/// (That corresponds to the Item_singlerow_subselect class.)
double init_cost_if_not_materialized{0.0};
/// Cost of evaluating the filter for all rows if all subqueries in
/// it have been materialized beforehand. If there are no subqueries
/// in the condition, equals cost_if_not_materialized.
double cost_if_materialized{0.0};
/// Cost of materializing all subqueries present in the filter.
/// If there are no subqueries in the condition, equals zero.
double cost_to_materialize{0.0};
};
/// Used internally by EstimateFilterCost() only.
void AddCost(THD *thd, const ContainedSubquery &subquery, double num_rows,
FilterCost *cost);
/**
Estimate the cost of evaluating “condition”, “num_rows” times.
This is a fairly rudimentary estimation, _but_ it includes the cost
of any subqueries that may be present and that need evaluation.
*/
FilterCost EstimateFilterCost(THD *thd, double num_rows, Item *condition,
const Query_block *outer_query_block);
/**
A cheaper overload of EstimateFilterCost() that assumes that all
contained subqueries have already been extracted (ie., it skips the
walking, which can be fairly expensive). This data is typically
computed by FindContainedSubqueries().
*/
inline FilterCost EstimateFilterCost(
THD *thd, double num_rows,
const Mem_root_array<ContainedSubquery> &contained_subqueries) {
FilterCost cost;
cost.cost_if_not_materialized = num_rows * kApplyOneFilterCost;
cost.cost_if_materialized = num_rows * kApplyOneFilterCost;
for (const ContainedSubquery &subquery : contained_subqueries) {
AddCost(thd, subquery, num_rows, &cost);
}
return cost;
}
double EstimateCostForRefAccess(THD *thd, TABLE *table, unsigned key_idx,
double num_output_rows);
void EstimateSortCost(AccessPath *path);
void EstimateMaterializeCost(THD *thd, AccessPath *path);
/**
Estimate costs and result row count for an aggregate operation.
@param[in,out] path The AGGREGATE path.
@param[in] query_block The Query_block to which 'path' belongs.
@param[in,out] trace Optimizer trace text.
*/
void EstimateAggregateCost(AccessPath *path, const Query_block *query_block,
std::string *trace = nullptr);
void EstimateDeleteRowsCost(AccessPath *path);
void EstimateUpdateRowsCost(AccessPath *path);
/// Estimate the costs and row count for a STREAM AccessPath.
void EstimateStreamCost(AccessPath *path);
/// Estimate the costs and row count for a LIMIT_OFFSET AccessPath.
void EstimateLimitOffsetCost(AccessPath *path);
/// Estimate the costs and row count for a WINDOW AccessPath.
void EstimateWindowCost(AccessPath *path);
inline double FindOutputRowsForJoin(double left_rows, double right_rows,
const JoinPredicate *edge) {
double fanout = right_rows * edge->selectivity;
if (edge->expr->type == RelationalExpression::LEFT_JOIN) {
// For outer joins, every outer row produces at least one row (if none
// are matching, we get a NULL-complemented row).
// Note that this can cause inconsistent row counts; see bug #33550360
// and/or JoinHypergraph::has_reordered_left_joins.
fanout = std::max(fanout, 1.0);
} else if (edge->expr->type == RelationalExpression::SEMIJOIN) {
// Semi- and antijoin estimation is pretty tricky, since we want isn't
// really selectivity; we want the probability that at least one row
// is matching, which is something else entirely. However, given that
// we only have selectivity to work with, we don't really have anything
// better than to estimate it as a normal join and cap the result
// at selectivity 1.0 (ie., each outer row generates at most one inner row).
// Note that this can cause inconsistent row counts; see bug #33550360 and
// CostingReceiver::has_semijoin_with_possibly_clamped_child.
fanout = std::min(fanout, 1.0);
} else if (edge->expr->type == RelationalExpression::ANTIJOIN) {
// Antijoin are estimated as simply the opposite of semijoin (see above),
// but wrongly estimating 0 rows (or, of course, a negative amount) could be
// really bad, so we assume at least 10% coming out as a fudge factor.
// It's better to estimate too high than too low here.
fanout = std::max(1.0 - fanout, 0.1);
}
return left_rows * fanout;
}
#endif // SQL_JOIN_OPTIMIZER_COST_MODEL_H_
|
