MySQL 8.0.30
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26/// @file
28/// This file contains the HashJoinRowBuffer class and related
29/// functions/classes.
31/// A HashJoinBuffer is a row buffer that can hold a certain amount of rows.
32/// The rows are stored in a hash table, which allows for constant-time lookup.
33/// The HashJoinBuffer maintains its own internal MEM_ROOT, where all of the
34/// data is allocated.
36/// The HashJoinBuffer contains an operand with rows from one or more tables,
37/// keyed on the value we join on. Consider the following trivial example:
39/// SELECT FROM t1 JOIN t2 ON (t1.key = t2.key);
41/// Let us say that the table "t2" is stored in a HashJoinBuffer. In this case,
42/// the hash table key will be the value found in "t2.key", since that is the
43/// join condition that belongs to t2. If we have multiple equalities, they
44/// will be concatenated together in order to form the hash table key. The hash
45/// table key is a std::string_view.
47/// In order to store a row, we use the function StoreFromTableBuffers. See the
48/// comments attached to the function for more details.
50/// The amount of memory a HashJoinBuffer instance can use is limited by the
51/// system variable "join_buffer_size". However, note that we check whether we
52/// have exceeded the memory limit _after_ we have inserted data into the row
53/// buffer. As such, we will probably use a little bit more memory than
54/// specified by join_buffer_size.
56/// The primary use case for these classes is, as the name implies,
57/// for implementing hash join.
59#include <stddef.h>
60#include <stdint.h>
61#include <memory>
62#include <vector>
64#include "extra/robin-hood-hashing/robin_hood.h"
65#include "field_types.h"
66#include "map_helpers.h"
67#include "my_alloc.h"
68#include "my_inttypes.h"
69#include "my_table_map.h"
70#include "prealloced_array.h"
72#include "sql/item_cmpfunc.h"
73#include "sql/pack_rows.h"
74#include "sql/table.h"
75#include "sql_string.h"
77class Field;
78class QEP_TAB;
80namespace hash_join_buffer {
82/// The key type for the hash structure in HashJoinRowBuffer.
84/// A key consists of the value from one or more columns, taken from the join
85/// condition(s) in the query. E.g., if the join condition is
86/// (t1.col1 = t2.col1 AND t1.col2 = t2.col2), the key is (col1, col2), with the
87/// two key parts concatenated together.
89/// What the data actually contains depends on the comparison context for the
90/// join condition. For instance, if the join condition is between a string
91/// column and an integer column, the comparison will be done in a string
92/// context, and thus the integers will be converted to strings before storing.
93/// So the data we store in the key are in some cases converted, so that we can
94/// hash and compare them byte-by-byte (i.e. decimals), while other types are
95/// already comparable byte-by-byte (i.e. integers), and thus stored as-is.
97/// Note that the key data can come from items as well as fields if the join
98/// condition is an expression. E.g. if the join condition is
99/// UPPER(t1.col1) = UPPER(t2.col1), the join key data will come from an Item
100/// instead of a Field.
102/// The Key class never takes ownership of the data. As such, the user must
103/// ensure that the data has the proper lifetime. When storing rows in the row
104/// buffer, the data must have the same lifetime as the row buffer itself.
105/// When using the Key class for lookups in the row buffer, the same lifetime is
106/// not needed; the key object is only needed when the lookup is done.
107using Key = std::string_view;
110 public:
111 // This is a marker from C++17 that signals to the container that
112 // operator() can be called with arguments of which one of the types
113 // differs from the container's key type (ImmutableStringWithLength),
114 // and thus enables map.find(Key). The type itself does not matter.
115 using is_transparent = void;
117 bool operator()(const Key &str1,
118 const ImmutableStringWithLength &other) const {
119 return str1 == other.Decode();
120 }
123 const ImmutableStringWithLength &str2) const {
124 return str1 == str2;
125 }
128// A row in the hash join buffer is the same as the Key class.
132 public:
133 // This is a marker from C++17 that signals to the container that
134 // operator() can be called with an argument that differs from the
135 // container's key type (ImmutableStringWithLength), and thus enables
136 // map.find(Key). The type itself does not matter.
137 using is_transparent = void;
140 return robin_hood::hash_bytes(, key.size());
141 }
144 std::string_view decoded = key.Decode();
145 return robin_hood::hash_bytes(, decoded.size());
146 }
149// A convenience form of LoadIntoTableBuffers() that also verifies the end
150// pointer for us.
152 BufferRow row);
154// A convenience form of the above that also decodes the LinkedImmutableString
155// for us.
162 public:
163 // Construct the buffer. Note that Init() must be called before the buffer can
164 // be used.
166 std::vector<HashJoinCondition> join_conditions,
167 size_t max_mem_available_bytes);
169 // Initialize the HashJoinRowBuffer so it is ready to store rows. This
170 // function can be called multiple times; subsequent calls will only clear the
171 // buffer for existing rows.
172 bool Init();
174 /// Store the row that is currently lying in the tables record buffers.
175 /// The hash map key is extracted from the join conditions that the row buffer
176 /// holds.
177 ///
178 /// @param thd the thread handler
179 /// @param reject_duplicate_keys If true, reject rows with duplicate keys.
180 /// If a row is rejected, the function will still return ROW_STORED.
181 /// @param store_rows_with_null_in_condition Whether to store rows where the
182 /// join conditions contains SQL NULL.
183 ///
184 /// @retval ROW_STORED the row was stored.
185 /// @retval BUFFER_FULL the row was stored, and the buffer is full.
186 /// @retval FATAL_ERROR an unrecoverable error occurred (most likely,
187 /// malloc failed). It is the caller's responsibility to call
188 /// my_error().
189 StoreRowResult StoreRow(THD *thd, bool reject_duplicate_keys,
190 bool store_rows_with_null_in_condition);
192 size_t size() const { return m_hash_map->size(); }
194 bool empty() const { return m_hash_map->empty(); }
196 bool inited() const { return m_hash_map != nullptr; }
198 using hash_map_type = robin_hood::unordered_flat_map<
201 using hash_map_iterator = hash_map_type::const_iterator;
203 hash_map_iterator find(const Key &key) const { return m_hash_map->find(key); }
205 hash_map_iterator begin() const { return m_hash_map->begin(); }
207 hash_map_iterator end() const { return m_hash_map->end(); }
210 assert(Initialized());
211 return m_last_row_stored;
212 }
214 bool Initialized() const { return m_hash_map.get() != nullptr; }
216 bool contains(const Key &key) const { return find(key) != end(); }
218 private:
219 const std::vector<HashJoinCondition> m_join_conditions;
221 // A row can consist of parts from different tables. This structure tells us
222 // which tables that are involved.
225 // The MEM_ROOT on which all of the hash table keys and values are allocated.
226 // The actual hash map is on the regular heap.
229 // A MEM_ROOT used only for storing the final row (possibly both key and
230 // value). The code assumes fairly deeply that inserting a row never fails, so
231 // when m_mem_root goes full (we set a capacity on it to ensure that the last
232 // allocated block does not get too big), we allocate the very last row on
233 // this MEM_ROOT and the signal fullness so that we can start spilling to
234 // disk.
237 // The hash table where the rows are stored.
238 std::unique_ptr<hash_map_type> m_hash_map;
240 // A buffer we can use when we are constructing a join key from a join
241 // condition. In order to avoid reallocating memory, the buffer never shrinks.
245 // The maximum size of the buffer, given in bytes.
248 // The last row that was stored in the hash table, or nullptr if the hash
249 // table is empty. We may have to put this row back into the tables' record
250 // buffers if we have a child iterator that expects the record buffers to
251 // contain the last row returned by the storage engine (the probe phase of
252 // hash join may put any row in the hash table in the tables' record buffer).
253 // See HashJoinIterator::BuildHashTable() for an example of this.
256 // Fetch the relevant fields from each table, and pack them into m_mem_root
257 // as a LinkedImmutableString where the “next” pointer points to “next_ptr”.
258 // If that does not work (capacity reached), pack into m_overflow_mem_root
259 // instead and set “full” to true. If _that_ does not work (fatally out
260 // of memory), returns nullptr. Otherwise, returns a pointer to the newly
261 // packed string.
263 LinkedImmutableString next_ptr, bool *full);
266} // namespace hash_join_buffer
Definition: field.h:573
The variant with length (ImmutableStringWithLength) stores the length as a Varint128 (similar to prot...
Definition: immutable_string.h:62
std::string_view Decode() const
Definition: immutable_string.h:129
LinkedImmutableString is designed for storing rows (values) in hash join.
Definition: immutable_string.h:172
Definition: sql_executor.h:259
Using this class is fraught with peril, and you need to be very careful when doing so.
Definition: sql_string.h:166
For each client connection we create a separate thread with THD serving as a thread/connection descri...
Definition: sql_class.h:922
Definition: hash_join_buffer.h:161
String m_buffer
Definition: hash_join_buffer.h:242
bool contains(const Key &key) const
Definition: hash_join_buffer.h:216
size_t m_row_size_upper_bound
Definition: hash_join_buffer.h:243
hash_map_type::const_iterator hash_map_iterator
Definition: hash_join_buffer.h:201
StoreRowResult StoreRow(THD *thd, bool reject_duplicate_keys, bool store_rows_with_null_in_condition)
Store the row that is currently lying in the tables record buffers.
hash_map_iterator end() const
Definition: hash_join_buffer.h:207
size_t size() const
Definition: hash_join_buffer.h:192
bool Init()
bool Initialized() const
Definition: hash_join_buffer.h:214
hash_map_iterator begin() const
Definition: hash_join_buffer.h:205
bool empty() const
Definition: hash_join_buffer.h:194
bool inited() const
Definition: hash_join_buffer.h:196
std::unique_ptr< hash_map_type > m_hash_map
Definition: hash_join_buffer.h:238
HashJoinRowBuffer(pack_rows::TableCollection tables, std::vector< HashJoinCondition > join_conditions, size_t max_mem_available_bytes)
LinkedImmutableString m_last_row_stored
Definition: hash_join_buffer.h:254
const std::vector< HashJoinCondition > m_join_conditions
Definition: hash_join_buffer.h:219
const size_t m_max_mem_available
Definition: hash_join_buffer.h:246
LinkedImmutableString StoreLinkedImmutableStringFromTableBuffers(LinkedImmutableString next_ptr, bool *full)
const pack_rows::TableCollection m_tables
Definition: hash_join_buffer.h:223
robin_hood::unordered_flat_map< ImmutableStringWithLength, LinkedImmutableString, KeyHasher, KeyEquals > hash_map_type
Definition: hash_join_buffer.h:199
LinkedImmutableString LastRowStored() const
Definition: hash_join_buffer.h:209
hash_map_iterator find(const Key &key) const
Definition: hash_join_buffer.h:203
MEM_ROOT m_overflow_mem_root
Definition: hash_join_buffer.h:235
MEM_ROOT m_mem_root
Definition: hash_join_buffer.h:227
Definition: hash_join_buffer.h:109
void is_transparent
Definition: hash_join_buffer.h:115
bool operator()(const ImmutableStringWithLength &str1, const ImmutableStringWithLength &str2) const
Definition: hash_join_buffer.h:122
bool operator()(const Key &str1, const ImmutableStringWithLength &other) const
Definition: hash_join_buffer.h:117
Definition: hash_join_buffer.h:131
size_t operator()(hash_join_buffer::Key key) const
Definition: hash_join_buffer.h:139
void is_transparent
Definition: hash_join_buffer.h:137
size_t operator()(ImmutableStringWithLength key) const
Definition: hash_join_buffer.h:143
A structure that contains a list of tables for the hash join operation, and some pre-computed propert...
Definition: pack_rows.h:83
This file contains the field type.
ImmutableString defines a storage format for strings that is designed to be as compact as possible,...
This file follows Google coding style, except for the name MEM_ROOT (which is kept for historical rea...
Some integer typedefs for easier portability.
std::string_view Key
The key type for the hash structure in HashJoinRowBuffer.
Definition: hash_join_buffer.h:107
Key BufferRow
Definition: hash_join_buffer.h:129
void LoadImmutableStringIntoTableBuffers(const TableCollection &tables, LinkedImmutableString row)
void LoadBufferRowIntoTableBuffers(const TableCollection &tables, BufferRow row)
Definition: hash_join_buffer.h:159
Generic routines for packing rows (possibly from multiple tables at the same time) into strings,...
required string key
Definition: replication_asynchronous_connection_failover.proto:59
Our own string classes, used pervasively throughout the executor.
The MEM_ROOT is a simple arena, where allocations are carved out of larger blocks.
Definition: my_alloc.h:82