テーブル結合を使用する Explain ステートメント
TiDB では、SQL オプティマイザーはテーブルを結合する順序と、特定の SQL ステートメントに対する最も効率的な結合アルゴリズムを決定する必要があります。このドキュメントの例は、次のサンプル データに基づいています。
CREATE TABLE t1 (id BIGINT NOT NULL PRIMARY KEY auto_increment, pad1 BLOB, pad2 BLOB, pad3 BLOB, int_col INT NOT NULL DEFAULT 0);
CREATE TABLE t2 (id BIGINT NOT NULL PRIMARY KEY auto_increment, t1_id BIGINT NOT NULL, pad1 BLOB, pad2 BLOB, pad3 BLOB, INDEX(t1_id));
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM dual;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
UPDATE t1 SET int_col = 1 WHERE pad1 = (SELECT pad1 FROM t1 ORDER BY RAND() LIMIT 1);
SELECT SLEEP(1);
ANALYZE TABLE t1, t2;
インデックス結合
結合する必要がある推定行数が少ない場合 (通常は 10000 行未満)、インデックス結合方法を使用することをお勧めします。この結合方法は、MySQL で使用される主な結合方法と同様に機能します。次の例では、オペレータ├─TableReader_29(Build)が最初にテーブルt1を読み取ります。一致する行ごとに、TiDB はテーブルを調査しますt2 :
注記：
返された実行プランでは、
IndexJoinおよびApply演算子のすべてのプローブ側子ノードについて、v6.4.0 以降のestRowsの意味は v6.4.0 以前とは異なります。詳細については、 TiDB クエリ実行計画の概要を参照してください。
EXPLAIN SELECT /*+ INL_JOIN(t1, t2) */ * FROM t1 INNER JOIN t2 ON t1.id = t2.t1_id;
+---------------------------------+----------+-----------+------------------------------+---------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------------+----------+-----------+------------------------------+---------------------------------------------------------------------------------------------------------------------------+
| IndexJoin_11 | 90000.00 | root | | inner join, inner:IndexLookUp_10, outer key:test.t1.id, inner key:test.t2.t1_id, equal cond:eq(test.t1.id, test.t2.t1_id) |
| ├─TableReader_29(Build) | 71010.00 | root | | data:TableFullScan_28 |
| │ └─TableFullScan_28 | 71010.00 | cop[tikv] | table:t1 | keep order:false |
| └─IndexLookUp_10(Probe) | 90000.00 | root | | |
| ├─IndexRangeScan_8(Build) | 90000.00 | cop[tikv] | table:t2, index:t1_id(t1_id) | range: decided by [eq(test.t2.t1_id, test.t1.id)], keep order:false |
| └─TableRowIDScan_9(Probe) | 90000.00 | cop[tikv] | table:t2 | keep order:false |
+---------------------------------+----------+-----------+------------------------------+---------------------------------------------------------------------------------------------------------------------------+
インデックス結合はメモリ使用量においては効率的ですが、多数のプローブ操作が必要な場合は他の結合方法よりも実行が遅くなる可能性があります。次のクエリも考慮してください。
SELECT * FROM t1 INNER JOIN t2 ON t1.id=t2.t1_id WHERE t1.pad1 = 'value' and t2.pad1='value';
内部結合操作では、TiDB は結合の並べ替えを実装し、最初にt1またはt2いずれかにアクセスする可能性があります。 TiDB がbuildステップを適用する最初のテーブルとしてt1を選択すると、TiDB はテーブルt2を調査する前に述語t1.col = 'value'でフィルタリングできるとします。述語t2.col='value'のフィルターはテーブルt2の各プローブに適用されますが、他の結合方法よりも効率が低い可能性があります。
インデックス結合は、ビルド側が小さく、プローブ側が事前にインデックス付けされていて大きい場合に効果的です。インデックス結合のパフォーマンスがハッシュ結合よりも悪く、SQL オプティマイザーによって選択されない次のクエリを考えてみましょう。
-- DROP previously added index
ALTER TABLE t2 DROP INDEX t1_id;
EXPLAIN ANALYZE SELECT /*+ INL_JOIN(t1, t2) */ * FROM t1 INNER JOIN t2 ON t1.id = t2.t1_id WHERE t1.int_col = 1;
EXPLAIN ANALYZE SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1 INNER JOIN t2 ON t1.id = t2.t1_id WHERE t1.int_col = 1;
EXPLAIN ANALYZE SELECT * FROM t1 INNER JOIN t2 ON t1.id = t2.t1_id WHERE t1.int_col = 1;
+-----------------------------+----------+---------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------+---------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+-----------------------------+----------+---------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------+---------+------+
| IndexJoin_14 | 90000.00 | 0 | root | | time:330.2ms, loops:1, inner:{total:72.2ms, concurrency:5, task:12, construct:58.6ms, fetch:13.5ms, build:2.12µs}, probe:26.1ms | inner join, inner:TableReader_10, outer key:test.t2.t1_id, inner key:test.t1.id, equal cond:eq(test.t2.t1_id, test.t1.id) | 88.5 MB | N/A |
| ├─TableReader_20(Build) | 90000.00 | 90000 | root | | time:307.2ms, loops:96, cop_task: {num: 24, max: 130.6ms, min: 170.9µs, avg: 33.5ms, p95: 105ms, max_proc_keys: 10687, p95_proc_keys: 9184, tot_proc: 472ms, rpc_num: 24, rpc_time: 802.4ms, copr_cache_hit_ratio: 0.62, distsql_concurrency: 15} | data:TableFullScan_19 | 58.6 MB | N/A |
| │ └─TableFullScan_19 | 90000.00 | 90000 | cop[tikv] | table:t2 | tikv_task:{proc max:34ms, min:0s, avg: 15.3ms, p80:24ms, p95:30ms, iters:181, tasks:24}, scan_detail: {total_process_keys: 69744, total_process_keys_size: 217533936, total_keys: 69753, get_snapshot_time: 701.6µs, rocksdb: {delete_skipped_count: 97368, key_skipped_count: 236847, block: {cache_hit_count: 3509}}} | keep order:false | N/A | N/A |
| └─TableReader_10(Probe) | 12617.92 | 0 | root | | time:11.9ms, loops:12, cop_task: {num: 42, max: 848.8µs, min: 199µs, avg: 451.8µs, p95: 846.2µs, max_proc_keys: 7, p95_proc_keys: 5, rpc_num: 42, rpc_time: 18.3ms, copr_cache_hit_ratio: 0.00, distsql_concurrency: 15} | data:Selection_9 | N/A | N/A |
| └─Selection_9 | 12617.92 | 0 | cop[tikv] | | tikv_task:{proc max:0s, min:0s, avg: 0s, p80:0s, p95:0s, iters:42, tasks:42}, scan_detail: {total_process_keys: 56, total_process_keys_size: 174608, total_keys: 77, get_snapshot_time: 727.7µs, rocksdb: {block: {cache_hit_count: 154}}} | eq(test.t1.int_col, 1) | N/A | N/A |
| └─TableRangeScan_8 | 90000.00 | 56 | cop[tikv] | table:t1 | tikv_task:{proc max:0s, min:0s, avg: 0s, p80:0s, p95:0s, iters:42, tasks:42} | range: decided by [test.t2.t1_id], keep order:false | N/A | N/A |
+-----------------------------+----------+---------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------+---------+------+
+------------------------------+----------+---------+-----------+---------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+------------------------------+----------+---------+-----------+---------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
| HashJoin_20 | 90000.00 | 0 | root | | time:313.6ms, loops:1, build_hash_table:{total:24.6ms, fetch:21.2ms, build:3.32ms}, probe:{concurrency:5, total:1.57s, max:313.5ms, probe:18.9ms, fetch:1.55s} | inner join, equal:[eq(test.t1.id, test.t2.t1_id)] | 32.0 MB | 0 Bytes |
| ├─TableReader_23(Build) | 9955.54 | 10000 | root | | time:23.6ms, loops:12, cop_task: {num: 11, max: 504.6µs, min: 203.7µs, avg: 377.4µs, p95: 504.6µs, rpc_num: 11, rpc_time: 3.92ms, copr_cache_hit_ratio: 1.00, distsql_concurrency: 15} | data:Selection_22 | 14.9 MB | N/A |
| │ └─Selection_22 | 9955.54 | 10000 | cop[tikv] | | tikv_task:{proc max:104ms, min:3ms, avg: 24.4ms, p80:33ms, p95:104ms, iters:113, tasks:11}, scan_detail: {get_snapshot_time: 241.4µs, rocksdb: {block: {}}} | eq(test.t1.int_col, 1) | N/A | N/A |
| │ └─TableFullScan_21 | 71010.00 | 71010 | cop[tikv] | table:t1 | tikv_task:{proc max:101ms, min:3ms, avg: 23.8ms, p80:33ms, p95:101ms, iters:113, tasks:11} | keep order:false | N/A | N/A |
| └─TableReader_25(Probe) | 90000.00 | 90000 | root | | time:293.7ms, loops:91, cop_task: {num: 24, max: 105.7ms, min: 210.9µs, avg: 31.4ms, p95: 103.8ms, max_proc_keys: 10687, p95_proc_keys: 9184, tot_proc: 407ms, rpc_num: 24, rpc_time: 752.2ms, copr_cache_hit_ratio: 0.62, distsql_concurrency: 15} | data:TableFullScan_24 | 58.6 MB | N/A |
| └─TableFullScan_24 | 90000.00 | 90000 | cop[tikv] | table:t2 | tikv_task:{proc max:31ms, min:0s, avg: 13ms, p80:19ms, p95:26ms, iters:181, tasks:24}, scan_detail: {total_process_keys: 69744, total_process_keys_size: 217533936, total_keys: 69753, get_snapshot_time: 637.2µs, rocksdb: {delete_skipped_count: 97368, key_skipped_count: 236847, block: {cache_hit_count: 3509}}} | keep order:false | N/A | N/A |
+------------------------------+----------+---------+-----------+---------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
+------------------------------+----------+---------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+------------------------------+----------+---------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
| HashJoin_21 | 90000.00 | 0 | root | | time:331.7ms, loops:1, build_hash_table:{total:32.7ms, fetch:26ms, build:6.73ms}, probe:{concurrency:5, total:1.66s, max:331.3ms, probe:16ms, fetch:1.64s} | inner join, equal:[eq(test.t1.id, test.t2.t1_id)] | 32.3 MB | 0 Bytes |
| ├─TableReader_26(Build) | 9955.54 | 10000 | root | | time:30.4ms, loops:13, cop_task: {num: 11, max: 1.87ms, min: 844.7µs, avg: 1.29ms, p95: 1.87ms, rpc_num: 11, rpc_time: 13.5ms, copr_cache_hit_ratio: 1.00, distsql_concurrency: 15} | data:Selection_25 | 12.2 MB | N/A |
| │ └─Selection_25 | 9955.54 | 10000 | cop[tikv] | | tikv_task:{proc max:104ms, min:3ms, avg: 24.4ms, p80:33ms, p95:104ms, iters:113, tasks:11}, scan_detail: {get_snapshot_time: 521µs, rocksdb: {block: {}}} | eq(test.t1.int_col, 1) | N/A | N/A |
| │ └─TableFullScan_24 | 71010.00 | 71010 | cop[tikv] | table:t1 | tikv_task:{proc max:101ms, min:3ms, avg: 23.8ms, p80:33ms, p95:101ms, iters:113, tasks:11} | keep order:false | N/A | N/A |
| └─TableReader_23(Probe) | 90000.00 | 90000 | root | | time:308.6ms, loops:91, cop_task: {num: 24, max: 123.3ms, min: 518.9µs, avg: 32.4ms, p95: 113.4ms, max_proc_keys: 10687, p95_proc_keys: 9184, tot_proc: 499ms, rpc_num: 24, rpc_time: 776ms, copr_cache_hit_ratio: 0.62, distsql_concurrency: 15} | data:TableFullScan_22 | 58.6 MB | N/A |
| └─TableFullScan_22 | 90000.00 | 90000 | cop[tikv] | table:t2 | tikv_task:{proc max:44ms, min:0s, avg: 16.8ms, p80:27ms, p95:40ms, iters:181, tasks:24}, scan_detail: {total_process_keys: 69744, total_process_keys_size: 217533936, total_keys: 69753, get_snapshot_time: 955.4µs, rocksdb: {delete_skipped_count: 97368, key_skipped_count: 236847, block: {cache_hit_count: 3509}}} | keep order:false | N/A | N/A |
+------------------------------+----------+---------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
上記の例では、インデックス結合操作でt1.int_colのインデックスが欠落しています。このインデックスを追加すると、次の結果が示すように、操作のパフォーマンスが0.3 secから0.06 secに向上します。
-- Re-add index
ALTER TABLE t2 ADD INDEX (t1_id);
EXPLAIN ANALYZE SELECT /*+ INL_JOIN(t1, t2) */ * FROM t1 INNER JOIN t2 ON t1.id = t2.t1_id WHERE t1.int_col = 1;
EXPLAIN ANALYZE SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1 INNER JOIN t2 ON t1.id = t2.t1_id WHERE t1.int_col = 1;
EXPLAIN ANALYZE SELECT * FROM t1 INNER JOIN t2 ON t1.id = t2.t1_id WHERE t1.int_col = 1;
+----------------------------------+----------+---------+-----------+------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------+-----------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+----------------------------------+----------+---------+-----------+------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------+-----------+------+
| IndexJoin_12 | 90000.00 | 0 | root | | time:65.6ms, loops:1, inner:{total:129.7ms, concurrency:5, task:7, construct:7.13ms, fetch:122.5ms, build:16.4µs}, probe:2.54ms | inner join, inner:IndexLookUp_11, outer key:test.t1.id, inner key:test.t2.t1_id, equal cond:eq(test.t1.id, test.t2.t1_id) | 28.7 MB | N/A |
| ├─TableReader_33(Build) | 9955.54 | 10000 | root | | time:15.4ms, loops:16, cop_task: {num: 11, max: 1.52ms, min: 211.5µs, avg: 416.8µs, p95: 1.52ms, rpc_num: 11, rpc_time: 4.36ms, copr_cache_hit_ratio: 1.00, distsql_concurrency: 15} | data:Selection_32 | 13.9 MB | N/A |
| │ └─Selection_32 | 9955.54 | 10000 | cop[tikv] | | tikv_task:{proc max:104ms, min:3ms, avg: 24.4ms, p80:33ms, p95:104ms, iters:113, tasks:11}, scan_detail: {get_snapshot_time: 185µs, rocksdb: {block: {}}} | eq(test.t1.int_col, 1) | N/A | N/A |
| │ └─TableFullScan_31 | 71010.00 | 71010 | cop[tikv] | table:t1 | tikv_task:{proc max:101ms, min:3ms, avg: 23.8ms, p80:33ms, p95:101ms, iters:113, tasks:11} | keep order:false | N/A | N/A |
| └─IndexLookUp_11(Probe) | 90000.00 | 0 | root | | time:115.6ms, loops:7 | | 555 Bytes | N/A |
| ├─IndexRangeScan_9(Build) | 90000.00 | 0 | cop[tikv] | table:t2, index:t1_id(t1_id) | time:114.3ms, loops:7, cop_task: {num: 7, max: 42ms, min: 1.3ms, avg: 16.2ms, p95: 42ms, tot_proc: 71ms, rpc_num: 7, rpc_time: 113.2ms, copr_cache_hit_ratio: 0.29, distsql_concurrency: 15}, tikv_task:{proc max:37ms, min:0s, avg: 11.3ms, p80:20ms, p95:37ms, iters:7, tasks:7}, scan_detail: {total_keys: 9296, get_snapshot_time: 141.9µs, rocksdb: {block: {cache_hit_count: 18592}}} | range: decided by [eq(test.t2.t1_id, test.t1.id)], keep order:false | N/A | N/A |
| └─TableRowIDScan_10(Probe) | 90000.00 | 0 | cop[tikv] | table:t2 | | keep order:false | N/A | N/A |
+----------------------------------+----------+---------+-----------+------------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------+-----------+------+
+------------------------------+----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+------------------------------+----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
| HashJoin_32 | 90000.00 | 0 | root | | time:320.2ms, loops:1, build_hash_table:{total:19.3ms, fetch:16.8ms, build:2.52ms}, probe:{concurrency:5, total:1.6s, max:320.1ms, probe:16.1ms, fetch:1.58s} | inner join, equal:[eq(test.t1.id, test.t2.t1_id)] | 32.0 MB | 0 Bytes |
| ├─TableReader_35(Build) | 9955.54 | 10000 | root | | time:18.6ms, loops:12, cop_task: {num: 11, max: 713.8µs, min: 197.3µs, avg: 368.5µs, p95: 713.8µs, rpc_num: 11, rpc_time: 3.83ms, copr_cache_hit_ratio: 1.00, distsql_concurrency: 15} | data:Selection_34 | 14.9 MB | N/A |
| │ └─Selection_34 | 9955.54 | 10000 | cop[tikv] | | tikv_task:{proc max:104ms, min:3ms, avg: 24.4ms, p80:33ms, p95:104ms, iters:113, tasks:11}, scan_detail: {get_snapshot_time: 178.9µs, rocksdb: {block: {}}} | eq(test.t1.int_col, 1) | N/A | N/A |
| │ └─TableFullScan_33 | 71010.00 | 71010 | cop[tikv] | table:t1 | tikv_task:{proc max:101ms, min:3ms, avg: 23.8ms, p80:33ms, p95:101ms, iters:113, tasks:11} | keep order:false | N/A | N/A |
| └─TableReader_37(Probe) | 90000.00 | 90000 | root | | time:304.4ms, loops:91, cop_task: {num: 24, max: 114ms, min: 251.1µs, avg: 33.1ms, p95: 110.4ms, max_proc_keys: 10687, p95_proc_keys: 9184, tot_proc: 492ms, rpc_num: 24, rpc_time: 793ms, copr_cache_hit_ratio: 0.62, distsql_concurrency: 15} | data:TableFullScan_36 | 58.6 MB | N/A |
| └─TableFullScan_36 | 90000.00 | 90000 | cop[tikv] | table:t2 | tikv_task:{proc max:38ms, min:3ms, avg: 14.1ms, p80:23ms, p95:35ms, iters:181, tasks:24}, scan_detail: {total_process_keys: 69744, total_process_keys_size: 217533936, total_keys: 139497, get_snapshot_time: 577.2µs, rocksdb: {delete_skipped_count: 44208, key_skipped_count: 253431, block: {cache_hit_count: 3527}}} | keep order:false | N/A | N/A |
+------------------------------+----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
+------------------------------+----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+------------------------------+----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
| HashJoin_33 | 90000.00 | 0 | root | | time:306.3ms, loops:1, build_hash_table:{total:20.5ms, fetch:17.1ms, build:3.45ms}, probe:{concurrency:5, total:1.53s, max:305.9ms, probe:17.1ms, fetch:1.51s} | inner join, equal:[eq(test.t1.id, test.t2.t1_id)] | 32.0 MB | 0 Bytes |
| ├─TableReader_42(Build) | 9955.54 | 10000 | root | | time:19.6ms, loops:12, cop_task: {num: 11, max: 1.07ms, min: 246.1µs, avg: 600µs, p95: 1.07ms, rpc_num: 11, rpc_time: 6.17ms, copr_cache_hit_ratio: 1.00, distsql_concurrency: 15} | data:Selection_41 | 19.7 MB | N/A |
| │ └─Selection_41 | 9955.54 | 10000 | cop[tikv] | | tikv_task:{proc max:104ms, min:3ms, avg: 24.4ms, p80:33ms, p95:104ms, iters:113, tasks:11}, scan_detail: {get_snapshot_time: 282.9µs, rocksdb: {block: {}}} | eq(test.t1.int_col, 1) | N/A | N/A |
| │ └─TableFullScan_40 | 71010.00 | 71010 | cop[tikv] | table:t1 | tikv_task:{proc max:101ms, min:3ms, avg: 23.8ms, p80:33ms, p95:101ms, iters:113, tasks:11} | keep order:false | N/A | N/A |
| └─TableReader_44(Probe) | 90000.00 | 90000 | root | | time:289.2ms, loops:91, cop_task: {num: 24, max: 108.2ms, min: 252.8µs, avg: 31.3ms, p95: 106.1ms, max_proc_keys: 10687, p95_proc_keys: 9184, tot_proc: 445ms, rpc_num: 24, rpc_time: 750.4ms, copr_cache_hit_ratio: 0.62, distsql_concurrency: 15} | data:TableFullScan_43 | 58.6 MB | N/A |
| └─TableFullScan_43 | 90000.00 | 90000 | cop[tikv] | table:t2 | tikv_task:{proc max:31ms, min:3ms, avg: 13.3ms, p80:24ms, p95:30ms, iters:181, tasks:24}, scan_detail: {total_process_keys: 69744, total_process_keys_size: 217533936, total_keys: 139497, get_snapshot_time: 730.2µs, rocksdb: {delete_skipped_count: 44208, key_skipped_count: 253431, block: {cache_hit_count: 3527}}} | keep order:false | N/A | N/A |
+------------------------------+----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------------------------------------------+---------+---------+
注記：
上記の例では、SQL オプティマイザーはインデックス結合よりもパフォーマンスの悪いハッシュ結合プランを選択します。クエリの最適化はNP完全問題であり、最適とはいえないプランが選択される可能性があります。これが頻繁なクエリの場合は、 SQL計画管理使用してヒントをクエリにバインドすることをお勧めします。これは、アプリケーションが TiDB に送信するクエリにヒントを挿入するよりも管理が簡単です。
インデックス結合のバリエーション
ヒントINL_JOINを使用したインデックス結合操作では、外部表に結合する前に中間結果のハッシュ テーブルが作成されます。 TiDB は、ヒントINL_HASH_JOINを使用した外部テーブルでのハッシュ テーブルの作成もサポートしています。これらのインデックス結合の各バリエーションは、SQL オプティマイザーによって自動的に選択されます。
コンフィグレーション
インデックス結合のパフォーマンスは、次のシステム変数の影響を受けます。
tidb_index_join_batch_size(デフォルト値:25000) -index lookup joinの操作のバッチ サイズ。tidb_index_lookup_join_concurrency(デフォルト値:4) - 同時インデックス検索タスクの数。
ハッシュ結合
ハッシュ結合操作では、TiDB は結合のBuild側のデータを読み取り、ハッシュ テーブルにキャッシュし、次に結合のProbe側のデータを読み取り、ハッシュ テーブルをプローブして必要な行にアクセスします。ハッシュ結合はインデックス結合よりも実行に多くのメモリを必要としますが、結合する必要がある行が多数ある場合は実行がはるかに速くなります。ハッシュ結合演算子は TiDB でマルチスレッドであり、並列実行されます。
ハッシュ結合の例は次のとおりです。
EXPLAIN SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
+-----------------------------+-----------+-----------+---------------+------------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+-----------+-----------+---------------+------------------------------------------------+
| HashJoin_27 | 142020.00 | root | | inner join, equal:[eq(test.t1.id, test.t2.id)] |
| ├─TableReader_29(Build) | 142020.00 | root | | data:TableFullScan_28 |
| │ └─TableFullScan_28 | 142020.00 | cop[tikv] | table:t1 | keep order:false |
| └─TableReader_31(Probe) | 180000.00 | root | | data:TableFullScan_30 |
| └─TableFullScan_30 | 180000.00 | cop[tikv] | table:t2 | keep order:false |
+-----------------------------+-----------+-----------+---------------+------------------------------------------------+
5 rows in set (0.00 sec)
HashJoin_27の実行プロセスでは、TiDB は次の操作を順番に実行します。
Build面のデータをメモリにキャッシュします。- キャッシュされたデータに基づいて
Build側でハッシュ テーブルを構築します。 Probe側のデータを読み出します。Probe側のデータを使用してハッシュ テーブルを調査します。- 修飾されたデータをユーザーに返します。
EXPLAIN結果テーブルのoperator info列には、クエリが内部結合か外部結合か、結合の条件など、 HashJoin_27に関するその他の情報も記録されます。上記の例では、クエリは内部結合であり、結合条件equal:[eq(test.t1.id, test.t2.id)]がクエリ条件WHERE t1.id = t2.idと部分的に一致します。次の例の他の結合演算子の演算子情報は、これと同様です。
実行時の統計
tidb_mem_quota_query (デフォルト値: 1 GB) を超え、 tidb_enable_tmp_storage_on_oom値がON (デフォルト) の場合、TiDB は一時storageの使用を試み、ディスク上にBuild演算子 (ハッシュ結合の一部として使用) を作成する可能性があります。メモリ使用量などの実行時の統計は、 EXPLAIN ANALYZE中execution infoの結果テーブルに記録されます。次の例は、 1 GB (デフォルト) とtidb_mem_quota_queryの 500 MB クォータを備えたEXPLAIN ANALYZEの出力を示しています。 500 MB では、ディスクが一時storageとして使用されます。
EXPLAIN ANALYZE SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
SET tidb_mem_quota_query=500 * 1024 * 1024;
EXPLAIN ANALYZE SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
+-----------------------------+-----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------------------------+-----------------------+---------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+-----------------------------+-----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------------------------+-----------------------+---------+
| HashJoin_27 | 142020.00 | 71010 | root | | time:647.508572ms, loops:72, build_hash_table:{total:579.254415ms, fetch:566.91012ms, build:12.344295ms}, probe:{concurrency:5, total:3.23315006s, max:647.520113ms, probe:330.884716ms, fetch:2.902265344s} | inner join, equal:[eq(test.t1.id, test.t2.id)] | 209.61642456054688 MB | 0 Bytes |
| ├─TableReader_29(Build) | 142020.00 | 71010 | root | | time:567.088247ms, loops:72, cop_task: {num: 2, max: 569.809411ms, min: 369.67451ms, avg: 469.74196ms, p95: 569.809411ms, max_proc_keys: 39245, p95_proc_keys: 39245, tot_proc: 400ms, rpc_num: 2, rpc_time: 939.447231ms, copr_cache_hit_ratio: 0.00} | data:TableFullScan_28 | 210.2100534439087 MB | N/A |
| │ └─TableFullScan_28 | 142020.00 | 71010 | cop[tikv] | table:t1 | proc max:64ms, min:48ms, p80:64ms, p95:64ms, iters:79, tasks:2 | keep order:false | N/A | N/A |
| └─TableReader_31(Probe) | 180000.00 | 90000 | root | | time:337.233636ms, loops:91, cop_task: {num: 3, max: 569.790741ms, min: 332.758911ms, avg: 421.543165ms, p95: 569.790741ms, max_proc_keys: 31719, p95_proc_keys: 31719, tot_proc: 500ms, rpc_num: 3, rpc_time: 1.264570696s, copr_cache_hit_ratio: 0.00} | data:TableFullScan_30 | 267.1126985549927 MB | N/A |
| └─TableFullScan_30 | 180000.00 | 90000 | cop[tikv] | table:t2 | proc max:84ms, min:72ms, p80:84ms, p95:84ms, iters:102, tasks:3 | keep order:false | N/A | N/A |
+-----------------------------+-----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------------------------+-----------------------+---------+
5 rows in set (0.65 sec)
Query OK, 0 rows affected (0.00 sec)
+-----------------------------+-----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------------------------+-----------------------+----------------------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+-----------------------------+-----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------------------------+-----------------------+----------------------+
| HashJoin_27 | 142020.00 | 71010 | root | | time:963.983353ms, loops:72, build_hash_table:{total:775.961447ms, fetch:503.789677ms, build:272.17177ms}, probe:{concurrency:5, total:4.805454793s, max:963.973133ms, probe:922.156835ms, fetch:3.883297958s} | inner join, equal:[eq(test.t1.id, test.t2.id)] | 93.53974533081055 MB | 210.7459259033203 MB |
| ├─TableReader_29(Build) | 142020.00 | 71010 | root | | time:504.062018ms, loops:72, cop_task: {num: 2, max: 509.276857ms, min: 402.66386ms, avg: 455.970358ms, p95: 509.276857ms, max_proc_keys: 39245, p95_proc_keys: 39245, tot_proc: 384ms, rpc_num: 2, rpc_time: 911.893237ms, copr_cache_hit_ratio: 0.00} | data:TableFullScan_28 | 210.20934200286865 MB | N/A |
| │ └─TableFullScan_28 | 142020.00 | 71010 | cop[tikv] | table:t1 | proc max:88ms, min:72ms, p80:88ms, p95:88ms, iters:79, tasks:2 | keep order:false | N/A | N/A |
| └─TableReader_31(Probe) | 180000.00 | 90000 | root | | time:363.058382ms, loops:91, cop_task: {num: 3, max: 412.659191ms, min: 358.489688ms, avg: 391.463008ms, p95: 412.659191ms, max_proc_keys: 31719, p95_proc_keys: 31719, tot_proc: 484ms, rpc_num: 3, rpc_time: 1.174326746s, copr_cache_hit_ratio: 0.00} | data:TableFullScan_30 | 267.11340618133545 MB | N/A |
| └─TableFullScan_30 | 180000.00 | 90000 | cop[tikv] | table:t2 | proc max:92ms, min:64ms, p80:92ms, p95:92ms, iters:102, tasks:3 | keep order:false | N/A | N/A |
+-----------------------------+-----------+---------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------------------------------------------+-----------------------+----------------------+
5 rows in set (0.98 sec)
コンフィグレーション
ハッシュ結合のパフォーマンスは、次のシステム変数の影響を受けます。
tidb_mem_quota_query(デフォルト値: 1GB) - クエリのメモリ割り当てを超過した場合、TiDB はメモリを節約するためにハッシュ結合のBuild演算子をディスクにスピルしようとします。tidb_hash_join_concurrency(デフォルト値:5) - 同時ハッシュ結合タスクの数。
マージ結合
マージ結合は、結合の両側がソートされた順序で読み取られる場合に適用される特別な種類の結合です。これは効率的なジッパー マージに似ていると言えます。結合のBuildとProbe側の両方でデータが読み取られるため、結合操作はストリーミング操作のように機能します。マージ結合はハッシュ結合よりもはるかに少ないメモリを必要としますが、並列実行されません。
以下は例です。
EXPLAIN SELECT /*+ MERGE_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
+-----------------------------+-----------+-----------+---------------+-------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+-----------+-----------+---------------+-------------------------------------------------------+
| MergeJoin_7 | 142020.00 | root | | inner join, left key:test.t1.id, right key:test.t2.id |
| ├─TableReader_12(Build) | 180000.00 | root | | data:TableFullScan_11 |
| │ └─TableFullScan_11 | 180000.00 | cop[tikv] | table:t2 | keep order:true |
| └─TableReader_10(Probe) | 142020.00 | root | | data:TableFullScan_9 |
| └─TableFullScan_9 | 142020.00 | cop[tikv] | table:t1 | keep order:true |
+-----------------------------+-----------+-----------+---------------+-------------------------------------------------------+
5 rows in set (0.00 sec)
マージ結合演算子の実行プロセスでは、TiDB は次の操作を実行します。
- 結合グループのすべてのデータを
Build側からメモリに読み取ります。 Probe面のデータを読み出します。Probe側のデータの各行がBuild側の完全な結合グループと一致するかどうかを比較します。同等の条件とは別に、非同等の条件があります。ここでの「一致」とは主に、等価でない条件が満たされているかどうかを確認することを指します。結合グループとは、すべての結合キーのうち同じ値を持つデータを指します。