MySQL 9.1.0
Source Code Documentation
index_merge.h
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23
24#ifndef SQL_RANGE_OPTIMIZER_INDEX_MERGE_H_
25#define SQL_RANGE_OPTIMIZER_INDEX_MERGE_H_
26
27#include <assert.h>
28
29#include "my_alloc.h"
32#include "sql/sql_list.h"
33
34class RowIterator;
35class String;
36class Unique;
37struct MY_BITMAP;
38struct TABLE;
39
40/*
41 IndexMergeIterator - index_merge access method quick select.
42
43 IndexMergeIterator uses
44 * IndexRangeScanIterators to get rows
45 * Unique class to remove duplicate rows
46
47 INDEX MERGE OPTIMIZER
48 Current implementation doesn't detect all cases where index_merge could
49 be used, in particular:
50 * index_merge will never be used if range scan is possible (even if
51 range scan is more expensive)
52
53 * index_merge+'using index' is not supported (this the consequence of
54 the above restriction)
55
56 * If WHERE part contains complex nested AND and OR conditions, some ways
57 to retrieve rows using index_merge will not be considered. The choice
58 of read plan may depend on the order of conjuncts/disjuncts in WHERE
59 part of the query, see comments near imerge_list_or_list and
60 SEL_IMERGE::or_sel_tree_with_checks functions for details.
61
62 * There is no "index_merge_ref" method (but index_merge on non-first
63 table in join is possible with 'range checked for each record').
64
65 See comments around SEL_IMERGE class and test_quick_select for more
66 details.
67
68 ROW RETRIEVAL ALGORITHM
69
70 index_merge uses Unique class for duplicates removal. index_merge takes
71 advantage of Clustered Primary Key (CPK) if the table has one.
72 The index_merge algorithm consists of two phases:
73
74 Phase 1 (implemented in IndexMergeIterator::prepare_unique):
75 prepare()
76 {
77 activate 'index only';
78 while(retrieve next row for non-CPK scan)
79 {
80 if (there is a CPK scan and row will be retrieved by it)
81 skip this row;
82 else
83 put its rowid into Unique;
84 }
85 deactivate 'index only';
86 }
87
88 Phase 2 (implemented as sequence of IndexMergeIterator::Read()
89 calls):
90
91 fetch()
92 {
93 retrieve all rows from row pointers stored in Unique;
94 free Unique;
95 retrieve all rows for CPK scan;
96 }
97*/
98
100 public:
101 // NOTE: Both pk_quick_select (if non-nullptr) and all children must be
102 // of the type IndexRangeScanIterator, possibly wrapped in a TimingIterator.
107 ~IndexMergeIterator() override;
108
109 bool Init() override;
110 int Read() override;
111
112 private:
114
115 /* used to get rows collected in Unique */
117
118 /* quick select that uses clustered primary key (NULL if none) */
120
121 /* range quick selects this index_merge read consists of */
123
124 /* true if this select is currently doing a clustered PK scan */
126
128};
129
130#endif // SQL_RANGE_OPTIMIZER_INDEX_MERGE_H_
Definition: index_merge.h:99
unique_ptr_destroy_only< RowIterator > pk_quick_select
Definition: index_merge.h:119
unique_ptr_destroy_only< RowIterator > read_record
Definition: index_merge.h:116
~IndexMergeIterator() override
Definition: index_merge.cc:65
Mem_root_array< unique_ptr_destroy_only< RowIterator > > m_children
Definition: index_merge.h:122
MEM_ROOT * mem_root
Definition: index_merge.h:127
bool doing_pk_scan
Definition: index_merge.h:125
int Read() override
Read a single row.
Definition: index_merge.cc:220
unique_ptr_destroy_only< Unique > unique
Definition: index_merge.h:113
IndexMergeIterator(THD *thd, MEM_ROOT *mem_root, TABLE *table, unique_ptr_destroy_only< RowIterator > pk_quick_select, Mem_root_array< unique_ptr_destroy_only< RowIterator > > children)
Definition: index_merge.cc:56
bool Init() override
Initialize or reinitialize the iterator.
Definition: index_merge.cc:103
A typesafe replacement for DYNAMIC_ARRAY.
Definition: mem_root_array.h:426
A context for reading through a single table using a chosen access method: index read,...
Definition: row_iterator.h:82
THD * thd() const
Definition: row_iterator.h:228
Using this class is fraught with peril, and you need to be very careful when doing so.
Definition: sql_string.h:167
For each client connection we create a separate thread with THD serving as a thread/connection descri...
Definition: sql_lexer_thd.h:36
Definition: row_iterator.h:234
TABLE * table() const
Definition: row_iterator.h:246
Unique – class for unique (removing of duplicates).
Definition: uniques.h:53
This file follows Google coding style, except for the name MEM_ROOT (which is kept for historical rea...
std::unique_ptr< T, Destroy_only< T > > unique_ptr_destroy_only
std::unique_ptr, but only destroying.
Definition: my_alloc.h:480
The MEM_ROOT is a simple arena, where allocations are carved out of larger blocks.
Definition: my_alloc.h:83
Definition: my_bitmap.h:43
Definition: table.h:1421