Group 和 Distinct 列的次序影响查询性能
一、概述
优化拥有大量的分组和去重列的SQL时，这些排序列的次序，也是可以优化的地方。
测试数据结构
kingbase=# select count(distinct txt1 ) txt1, avg(length(txt1))::int ln1, count(distinct txt3 ) txt3 ,avg(length(txt3))::int ln3 from txt01;
txt1 | ln1 | txt3 | ln3
------+------+---------+-----
1000 | 1000 | 1000000 | 10
(1 行记录)
二、work_mem 满足排序情况
1、Distinct 语句
次序： txt1,txt3
kingbase=# explain (analyse ,buffers ) select /*+ set(work_mem 100MB) */ distinct txt1 ,txt3 from txt01 ;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=269287.33..269687.33 rows=40000 width=64) (actual time=1543.995..1877.527 rows=1000000 loops=1)
Group Key: txt1, txt3
Buffers: shared hit=142858
-> Seq Scan on txt01 (cost=0.00..227144.22 rows=8428622 width=64) (actual time=0.008..159.858 rows=1000000 loops=1)
Buffers: shared hit=142858
Planning Time: 0.081 ms
Execution Time: 1947.951 ms
(7 行记录)
次序： txt3,txt1
ingbase=# explain (analyse ,buffers ) select /*+ set(work_mem 100MB) */ distinct txt3 ,txt1 from txt01 ;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=269287.33..269687.33 rows=40000 width=64) (actual time=1596.040..1812.380 rows=1000000 loops=1)
Group Key: txt3, txt1
Buffers: shared hit=142858
-> Seq Scan on txt01 (cost=0.00..227144.22 rows=8428622 width=64) (actual time=0.007..163.399 rows=1000000 loops=1)
Buffers: shared hit=142858
Planning Time: 0.075 ms
Execution Time: 1884.907 ms
(7 行记录)
2、Group by 语句
次序： txt1,txt3
kingbase=# explain (analyse ,buffers ) select /*+ set(work_mem 100MB) */ txt1 ,txt3 from txt01 group by txt1 ,txt3 ;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=269287.33..269687.33 rows=40000 width=64) (actual time=1540.948..1875.917 rows=1000000 loops=1)
Group Key: txt1, txt3
Buffers: shared hit=142858
-> Seq Scan on txt01 (cost=0.00..227144.22 rows=8428622 width=64) (actual time=0.006..160.419 rows=1000000 loops=1)
Buffers: shared hit=142858
Planning Time: 0.084 ms
Execution Time: 1939.103 ms
(7 行记录)
次序： txt3,txt1
kingbase=# explain (analyse ,buffers ) select /*+ set(work_mem 100MB) */ txt1 ,txt3 from txt01 group by txt3 ,txt1 ;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=269287.33..269687.33 rows=40000 width=64) (actual time=1557.257..1780.662 rows=1000000 loops=1)
Group Key: txt3, txt1
Buffers: shared hit=142858
-> Seq Scan on txt01 (cost=0.00..227144.22 rows=8428622 width=64) (actual time=0.018..165.221 rows=1000000 loops=1)
Buffers: shared hit=142858
Planning Time: 0.330 ms
Execution Time: 1844.664 ms
(7 行记录)
三、work_mem 不满足排序情况
1、Distinct 语句
次序： txt1,txt3
kingbase=# explain (analyse ,buffers ) select /*+ set(work_mem 1MB) */ distinct txt1 ,txt3 from txt01 ;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Unique (cost=2464313.08..2527527.74 rows=40000 width=64) (actual time=21031.092..22131.259 rows=1000000 loops=1)
Buffers: shared hit=142858, temp read=125368 written=125369
-> Sort (cost=2464313.08..2485384.63 rows=8428622 width=64) (actual time=21031.089..22002.850 rows=1000000 loops=1)
Sort Key: txt1, txt3
Sort Method: external merge Disk: 1002944kB
Buffers: shared hit=142858, temp read=125368 written=125369
-> Seq Scan on txt01 (cost=0.00..227144.22 rows=8428622 width=64) (actual time=0.039..272.327 rows=1000000 loops=1)
Buffers: shared hit=142858
Planning Time: 0.272 ms
Execution Time: 23648.185 ms
(10 行记录)
次序： txt3,txt1
kingbase=# explain (analyse ,buffers ) select /*+ set(work_mem 1MB) */ distinct txt3 ,txt1 from txt01 ;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Unique (cost=2464313.08..2527527.74 rows=40000 width=64) (actual time=4004.641..4367.218 rows=1000000 loops=1)
Buffers: shared hit=142858, temp read=125491 written=125492
-> Sort (cost=2464313.08..2485384.63 rows=8428622 width=64) (actual time=4004.639..4239.599 rows=1000000 loops=1)
Sort Key: txt3, txt1
Sort Method: external merge Disk: 1003928kB
Buffers: shared hit=142858, temp read=125491 written=125492
-> Seq Scan on txt01 (cost=0.00..227144.22 rows=8428622 width=64) (actual time=0.011..271.572 rows=1000000 loops=1)
Buffers: shared hit=142858
Planning Time: 0.086 ms
Execution Time: 4457.751 ms
(10 行记录)
2、Group by 语句
次序： txt1,txt3
kingbase=# explain (analyse ,buffers ) select /*+ set(work_mem 1MB) */ txt1 ,txt3 from txt01 group by txt1 ,txt3 ;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Group (cost=2464313.08..2527527.74 rows=40000 width=64) (actual time=21715.770..22796.166 rows=1000000 loops=1)
Group Key: txt1, txt3
Buffers: shared hit=142858, temp read=125368 written=125369
-> Sort (cost=2464313.08..2485384.63 rows=8428622 width=64) (actual time=21715.764..22658.413 rows=1000000 loops=1)
Sort Key: txt1, txt3
Sort Method: external merge Disk: 1002944kB
Buffers: shared hit=142858, temp read=125368 written=125369
-> Seq Scan on txt01 (cost=0.00..227144.22 rows=8428622 width=64) (actual time=0.029..271.335 rows=1000000 loops=1)
Buffers: shared hit=142858
Planning Time: 0.285 ms
Execution Time: 25365.012 ms
(11 行记录)
次序： txt3,txt1
kingbase=# explain (analyse ,buffers ) select /*+ set(work_mem 1MB) */ txt1 ,txt3 from txt01 group by txt3 ,txt1 ;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Group (cost=2464313.08..2527527.74 rows=40000 width=64) (actual time=4156.296..4541.315 rows=1000000 loops=1)
Group Key: txt3, txt1
Buffers: shared hit=142858, temp read=125368 written=125369
-> Sort (cost=2464313.08..2485384.63 rows=8428622 width=64) (actual time=4156.291..4402.265 rows=1000000 loops=1)
Sort Key: txt3, txt1
Sort Method: external merge Disk: 1002944kB
Buffers: shared hit=142858, temp read=125368 written=125369
-> Seq Scan on txt01 (cost=0.00..227144.22 rows=8428622 width=64) (actual time=0.008..270.872 rows=1000000 loops=1)
Buffers: shared hit=142858
Planning Time: 0.081 ms
Execution Time: 4632.567 ms
(11 行记录)
四、总结
| 次序 | txt1,txt1 | txt3,txt1 |
|---|---|---|
| work_mem满足排序 | 1947.951 ms | 1884.907 ms |
| work_mem不足排序 | 25365.012 ms | 4632.567 ms |
字节少数据值多的列，处于排序列的前列，可以带来性能的提升。当work_mem满足排序时，性能差异不大，当work_mem不足时，性能提升较大。
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