【发布时间】：2021-12-19 17:08:05
【问题描述】：
我正在尝试加快我的 PostgreSQL 报告查询速度。我简化了数据模型以便更好地理解。
我的报告查询使用以下表格：
- 1 个大表，约 500.000 行。这很容易增长到数百万行。
- ~20 个小表，每行~20 行。它们只包含添加到大表中的文本。
大表如下所示：
CREATE TABLE big_table (
id character varying(100) NOT NULL,
attr_1_id character varying(100) NOT NULL,
attr_2_id character varying(100) NOT NULL,
attr_3_id character varying(100) NOT NULL,
...
CONSTRAINT big_table_pkey PRIMARY KEY (id)
);
小桌子看起来都是这样的：
CREATE TABLE small_table_1 (
id character varying(100) NOT NULL,
language_id character(2) NOT NULL,
text character varying(100),
CONSTRAINT small_table_1_pkey PRIMARY KEY (id)
);
查询如下所示：
SELECT
big_table.attr_1_id,
small_table_1.text,
big_table.attr_2_id,
small_table_2.text,
big_table.attr_3_id,
small_table_3.text,
...
FROM big_table
LEFT JOIN small_table_1
ON small_table_1.id = big_table.attr_id_1
AND small_table_1.language_id = 'DE'
LEFT JOIN small_table_2
ON small_table_2.id = big_table.attr_id_2
AND small_table_2.language_id = 'DE'
LEFT JOIN small_table_3
ON small_table_3.id = big_table.attr_id_3
AND small_table_3.language_id = 'DE'
...
每个LEFT JOIN 在大表中有 ~500.000 行已经花费了 ~200 毫秒。
因此，在加入数百万行和许多小表的情况下，执行时间会迅速增长。
在查询规划器中，每个 LEFT JOIN 看起来像这样：
Hash Left Join (cost=31.94..13727.27 rows=432242 width=148) (actual time=4.398..581.194 rows=432242 loops=1)
Hash Cond: ((small_table_1.id)::text = (big_table.attr_id_1)::text)
这是完整的真实查询计划。我仍然必须删除 10 个小表的 LEFT JOINs，以免达到 Stack Overflow 最大字符数限制 30,000。
"QUERY PLAN"
"Hash Left Join (cost=208.64..400076.38 rows=460222 width=3163) (actual time=7.508..7178.265 rows=493850 loops=1)"
" Hash Cond: ((""*SELECT* 1"".employment_status_id)::text = (estx.id)::text)"
" Buffers: shared hit=8290"
" CTE q2"
" -> Seq Scan on cust_group (cost=0.00..13.00 rows=300 width=16) (actual time=0.002..0.004 rows=1 loops=1)"
" Buffers: shared hit=1"
" InitPlan 2 (returns $1)"
" -> CTE Scan on q2 (cost=0.00..6.00 rows=300 width=16) (actual time=0.001..0.001 rows=1 loops=1)"
" InitPlan 3 (returns $2)"
" -> CTE Scan on q2 q2_1 (cost=0.00..6.00 rows=300 width=16) (actual time=0.000..0.000 rows=1 loops=1)"
" InitPlan 4 (returns $3)"
" -> CTE Scan on q2 q2_2 (cost=0.00..6.00 rows=300 width=16) (actual time=0.000..0.001 rows=1 loops=1)"
" InitPlan 5 (returns $4)"
" -> CTE Scan on q2 q2_3 (cost=0.00..6.00 rows=300 width=16) (actual time=0.004..0.005 rows=1 loops=1)"
" Buffers: shared hit=1"
" InitPlan 6 (returns $5)"
" -> CTE Scan on q2 q2_4 (cost=0.00..6.00 rows=300 width=16) (actual time=0.000..0.001 rows=1 loops=1)"
" InitPlan 7 (returns $6)"
" -> CTE Scan on q2 q2_5 (cost=0.00..6.00 rows=300 width=16) (actual time=0.000..0.000 rows=1 loops=1)"
" -> Hash Left Join (cost=158.57..338963.78 rows=460222 width=2513) (actual time=6.695..4724.369 rows=493850 loops=1)"
" Hash Cond: (((NULL::character varying))::text = (citx.id)::text)"
" Buffers: shared hit=8289"
" -> Hash Left Join (cost=156.37..337727.90 rows=460222 width=2490) (actual time=5.361..4492.058 rows=493850 loops=1)"
" Hash Cond: (""*SELECT* 1"".action_type_id = attx.id)"
" Buffers: shared hit=8288"
" -> Hash Left Join (cost=155.00..336492.84 rows=460222 width=2478) (actual time=3.754..4261.646 rows=493850 loops=1)"
" Hash Cond: ((""*SELECT* 1"".job_id)::text = (jbtx.id)::text)"
" Buffers: shared hit=8287"
" -> Hash Left Join (cost=128.25..335232.41 rows=460222 width=2457) (actual time=1.704..4048.729 rows=493850 loops=1)"
" Hash Cond: ((""*SELECT* 1"".employee_type_id)::text = (ettx.id)::text)"
" Buffers: shared hit=8264"
" -> Nested Loop Left Join (cost=126.96..333997.44 rows=460222 width=2239) (actual time=0.642..3788.566 rows=493850 loops=1)"
" Buffers: shared hit=8263"
" -> Nested Loop Left Join (cost=126.81..328236.49 rows=460222 width=2222) (actual time=0.622..3478.490 rows=493850 loops=1)"
" Buffers: shared hit=8261"
" -> Hash Left Join (cost=126.67..322475.55 rows=460222 width=2205) (actual time=0.612..3155.816 rows=493850 loops=1)"
" Hash Cond: ((""*SELECT* 1"".org_unit_id)::text = (outx.id)::text)"
" Buffers: shared hit=8259"
" -> Hash Left Join (cost=98.98..321214.18 rows=460222 width=2185) (actual time=0.541..2969.331 rows=493850 loops=1)"
" Hash Cond: ((""*SELECT* 1"".cost_center_id)::text = (cctx.id)::text)"
" Buffers: shared hit=8236"
" -> Hash Left Join (cost=96.93..319978.45 rows=460222 width=2175) (actual time=0.519..2712.470 rows=493850 loops=1)"
" Hash Cond: (coat.country_id = (cutx.id)::bpchar)"
" Buffers: shared hit=8235"
" -> Hash Left Join (cost=83.59..313061.78 rows=460222 width=2163) (actual time=0.412..2404.622 rows=493850 loops=1)"
" Hash Cond: ((""*SELECT* 1"".company_id)::text = (cotx.id)::text)"
" Buffers: shared hit=8231"
" -> Hash Left Join (cost=71.45..311815.96 rows=460222 width=1945) (actual time=0.382..2131.789 rows=493850 loops=1)"
" Hash Cond: ((NULL::bpchar) = (crtx_le.id)::bpchar)"
" Buffers: shared hit=8230"
" -> Nested Loop Left Join (cost=61.14..305017.38 rows=460222 width=1928) (actual time=0.298..1886.031 rows=493850 loops=1)"
" Buffers: shared hit=8227"
" -> Hash Left Join (cost=60.99..299256.43 rows=460222 width=1896) (actual time=0.282..1584.799 rows=493850 loops=1)"
" Hash Cond: ((""*SELECT* 1"".employee_id)::text = (emat.id)::text)"
" Buffers: shared hit=8225"
" -> Hash Left Join (cost=47.75..298019.21 rows=460222 width=1871) (actual time=0.131..1287.387 rows=493850 loops=1)"
" Hash Cond: (""*SELECT* 1"".scenario_id = scen.id)"
" Buffers: shared hit=8219"
" -> Hash Left Join (cost=46.70..296784.48 rows=460222 width=1853) (actual time=0.122..1026.567 rows=493850 loops=1)"
" Hash Cond: (""*SELECT* 1"".action_id = aclg.action_id)"
" Buffers: shared hit=8218"
" -> Hash Left Join (cost=38.02..292886.93 rows=460222 width=1789) (actual time=0.046..883.004 rows=493850 loops=1)"
" Hash Cond: ((""*SELECT* 1"".company_id)::text = (coat.id)::text)"
" Buffers: shared hit=8212"
" -> Hash Left Join (cost=21.05..291645.36 rows=460222 width=1773) (actual time=0.035..712.011 rows=493850 loops=1)"
" Hash Cond: ((NULL::bpchar) = (conv_gr.from_currency_id)::bpchar)"
" Buffers: shared hit=8211"
" -> Append (cost=0.00..285204.21 rows=460222 width=1742) (actual time=0.013..465.417 rows=493850 loops=1)"
" Buffers: shared hit=8209"
" -> Subquery Scan on ""*SELECT* 1"" (cost=0.00..2739.92 rows=68841 width=178) (actual time=0.012..55.052 rows=68841 loops=1)"
" Buffers: shared hit=1191"
" -> Seq Scan on appl_plan_headcount (cost=0.00..1879.41 rows=68841 width=150) (actual time=0.010..32.481 rows=68841 loops=1)"
" Buffers: shared hit=1191"
" -> Subquery Scan on ""*SELECT* 2"" (cost=3405.85..280163.18 rows=391381 width=151) (actual time=36.277..350.251 rows=425009 loops=1)"
" Buffers: shared hit=7018"
" -> Gather (cost=3405.85..274292.47 rows=391381 width=95) (actual time=36.275..231.496 rows=425009 loops=1)"
" Workers Planned: 2"
" Workers Launched: 2"
" Buffers: shared hit=7018"
" -> Parallel Hash Join (cost=2405.85..234154.37 rows=163075 width=95) (actual time=26.658..228.421 rows=141670 loops=3)"
" Hash Cond: ((co.scenario_id = hc.scenario_id) AND ((co.employee_id)::text = (hc.employee_id)::text) AND (co.cal_year_month_id = hc.cal_year_month_id) AND (co.action_id = hc.action_id))"
" Buffers: shared hit=7018"
" -> Parallel Seq Scan on appl_plan_costs co (cost=0.00..7464.01 rows=180101 width=45) (actual time=0.007..52.792 rows=144081 loops=3)"
" Buffers: shared hit=5663"
" -> Parallel Hash (cost=1595.95..1595.95 rows=40495 width=62) (actual time=25.189..25.189 rows=22947 loops=3)"
" Buckets: 131072 Batches: 1 Memory Usage: 8096kB"
" Buffers: shared hit=1191"
" -> Parallel Seq Scan on appl_plan_headcount hc (cost=0.00..1595.95 rows=40495 width=62) (actual time=0.009..10.810 rows=22947 loops=3)"
" Buffers: shared hit=1191"
" -> Hash (cost=21.00..21.00 rows=4 width=48) (actual time=0.016..0.017 rows=6 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 9kB"
" Buffers: shared hit=2"
" -> Seq Scan on cust_currency_conversion conv_gr (cost=0.00..21.00 rows=4 width=48) (actual time=0.011..0.013 rows=6 loops=1)"
" Filter: ((to_currency_id)::text = ($4)::text)"
" Rows Removed by Filter: 6"
" Buffers: shared hit=2"
" -> Hash (cost=13.10..13.10 rows=310 width=234) (actual time=0.006..0.006 rows=8 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 9kB"
" Buffers: shared hit=1"
" -> Seq Scan on appl_company_attr coat (cost=0.00..13.10 rows=310 width=234) (actual time=0.003..0.004 rows=8 loops=1)"
" Buffers: shared hit=1"
" -> Hash (cost=7.50..7.50 rows=94 width=72) (actual time=0.070..0.072 rows=91 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 17kB"
" Buffers: shared hit=6"
" -> Hash Left Join (cost=3.18..7.50 rows=94 width=72) (actual time=0.016..0.053 rows=91 loops=1)"
" Hash Cond: ((aclg.created_by)::text = (ausr.id)::text)"
" Buffers: shared hit=6"
" -> Seq Scan on appl_plan_action_log aclg (cost=0.00..3.94 rows=94 width=58) (actual time=0.003..0.013 rows=91 loops=1)"
" Buffers: shared hit=3"
" -> Hash (cost=3.08..3.08 rows=8 width=24) (actual time=0.008..0.009 rows=5 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 9kB"
" Buffers: shared hit=3"
" -> Seq Scan on auth_user ausr (cost=0.00..3.08 rows=8 width=24) (actual time=0.003..0.007 rows=5 loops=1)"
" Buffers: shared hit=3"
" -> Hash (cost=1.02..1.02 rows=2 width=22) (actual time=0.005..0.005 rows=4 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 9kB"
" Buffers: shared hit=1"
" -> Seq Scan on appl_scenarios scen (cost=0.00..1.02 rows=2 width=22) (actual time=0.003..0.003 rows=4 loops=1)"
" Buffers: shared hit=1"
" -> Hash (cost=9.22..9.22 rows=322 width=31) (actual time=0.146..0.146 rows=370 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 33kB"
" Buffers: shared hit=6"
" -> Seq Scan on appl_employee_attr emat (cost=0.00..9.22 rows=322 width=31) (actual time=0.005..0.057 rows=370 loops=1)"
" Buffers: shared hit=6"
" -> Materialize (cost=0.15..8.18 rows=1 width=32) (actual time=0.000..0.000 rows=1 loops=493850)"
" Buffers: shared hit=2"
" -> Index Scan using cust_currency_conversion_pkey on cust_currency_conversion conv_ta (cost=0.15..8.17 rows=1 width=32) (actual time=0.012..0.013 rows=1 loops=1)"
" Index Cond: (((from_currency_id)::text = ($5)::text) AND ((to_currency_id)::text = 'USD'::text))"
" Buffers: shared hit=2"
" -> Hash (cost=7.88..7.88 rows=195 width=21) (actual time=0.080..0.081 rows=195 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 19kB"
" Buffers: shared hit=3"
" -> Seq Scan on appl_currency_text crtx_le (cost=0.00..7.88 rows=195 width=21) (actual time=0.007..0.051 rows=195 loops=1)"
" Filter: (language_id = 'DE'::bpchar)"
" Rows Removed by Filter: 195"
" Buffers: shared hit=3"
" -> Hash (cost=12.12..12.12 rows=1 width=436) (actual time=0.007..0.007 rows=8 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 9kB"
" Buffers: shared hit=1"
" -> Seq Scan on appl_company_text cotx (cost=0.00..12.12 rows=1 width=436) (actual time=0.003..0.005 rows=8 loops=1)"
" Filter: (language_id = 'DE'::bpchar)"
" Rows Removed by Filter: 8"
" Buffers: shared hit=1"
" -> Hash (cost=10.23..10.23 rows=249 width=16) (actual time=0.102..0.102 rows=249 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 20kB"
" Buffers: shared hit=4"
" -> Seq Scan on appl_country_text cutx (cost=0.00..10.23 rows=249 width=16) (actual time=0.006..0.064 rows=249 loops=1)"
" Filter: (language_id = 'DE'::bpchar)"
" Rows Removed by Filter: 249"
" Buffers: shared hit=4"
" -> Hash (cost=1.70..1.70 rows=28 width=20) (actual time=0.015..0.016 rows=28 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 10kB"
" Buffers: shared hit=1"
" -> Seq Scan on appl_cost_center_text cctx (cost=0.00..1.70 rows=28 width=20) (actual time=0.003..0.010 rows=28 loops=1)"
" Filter: (language_id = 'DE'::bpchar)"
" Rows Removed by Filter: 28"
" Buffers: shared hit=1"
" -> Hash (cost=26.12..26.12 rows=125 width=29) (actual time=0.065..0.066 rows=125 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 16kB"
" Buffers: shared hit=23"
" -> Seq Scan on appl_org_unit_text outx (cost=0.00..26.12 rows=125 width=29) (actual time=0.004..0.046 rows=125 loops=1)"
" Filter: (language_id = 'DE'::bpchar)"
" Rows Removed by Filter: 125"
" Buffers: shared hit=23"
" -> Materialize (cost=0.15..8.17 rows=1 width=17) (actual time=0.000..0.000 rows=1 loops=493850)"
" Buffers: shared hit=2"
" -> Index Scan using appl_currency_text_pkey on appl_currency_text crtx_gr (cost=0.15..8.17 rows=1 width=17) (actual time=0.007..0.008 rows=1 loops=1)"
" Index Cond: (((id)::text = ($6)::text) AND (language_id = 'DE'::bpchar))"
" Buffers: shared hit=2"
" -> Materialize (cost=0.15..8.17 rows=1 width=17) (actual time=0.000..0.000 rows=1 loops=493850)"
" Buffers: shared hit=2"
" -> Index Scan using appl_currency_text_pkey on appl_currency_text crtx_ta (cost=0.15..8.17 rows=1 width=17) (actual time=0.007..0.010 rows=1 loops=1)"
" Index Cond: (((id)::text = 'USD'::text) AND (language_id = 'DE'::bpchar))"
" Buffers: shared hit=2"
" -> Hash (cost=1.27..1.27 rows=1 width=436) (actual time=0.503..0.504 rows=11 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 9kB"
" Buffers: shared hit=1"
" -> Seq Scan on appl_employee_type_text ettx (cost=0.00..1.27 rows=1 width=436) (actual time=0.007..0.010 rows=11 loops=1)"
" Filter: (language_id = 'DE'::bpchar)"
" Rows Removed by Filter: 11"
" Buffers: shared hit=1"
" -> Hash (cost=25.50..25.50 rows=100 width=29) (actual time=0.706..0.707 rows=100 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 14kB"
" Buffers: shared hit=23"
" -> Seq Scan on appl_job_text jbtx (cost=0.00..25.50 rows=100 width=29) (actual time=0.004..0.047 rows=100 loops=1)"
" Filter: (language_id = 'DE'::bpchar)"
" Rows Removed by Filter: 100"
" Buffers: shared hit=23"
" -> Hash (cost=1.25..1.25 rows=10 width=16) (actual time=0.409..0.410 rows=10 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 9kB"
" Buffers: shared hit=1"
" -> Seq Scan on appl_action_type_text attx (cost=0.00..1.25 rows=10 width=16) (actual time=0.006..0.008 rows=10 loops=1)"
" Filter: (language_id = 'DE'::bpchar)"
" Rows Removed by Filter: 10"
" Buffers: shared hit=1"
" -> Hash (cost=1.80..1.80 rows=32 width=26) (actual time=0.360..0.360 rows=32 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 10kB"
" Buffers: shared hit=1"
" -> Seq Scan on appl_cost_item_text citx (cost=0.00..1.80 rows=32 width=26) (actual time=0.008..0.015 rows=32 loops=1)"
" Filter: (language_id = 'DE'::bpchar)"
" Rows Removed by Filter: 32"
" Buffers: shared hit=1"
" -> Hash (cost=1.05..1.05 rows=1 width=296) (actual time=0.418..0.419 rows=2 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 9kB"
" Buffers: shared hit=1"
" -> Seq Scan on appl_employment_status_text estx (cost=0.00..1.05 rows=1 width=296) (actual time=0.007..0.008 rows=2 loops=1)"
" Filter: (language_id = 'DE'::bpchar)"
" Rows Removed by Filter: 2"
" Buffers: shared hit=1"
"Planning Time: 2.079 ms"
"Execution Time: 7247.830 ms"
几乎 80% 的时间花在LEFT JOINs 上。
更多信息：
- x86_64-pc-linux-gnu 上的 PostgreSQL 12.7，由 gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11) 编译，64 位
- 我配置了足够多的
work_mem，所以光盘上没有任何反应。 - 根据查询规划器，使用了
Hash Left Join，这应该是最快的。 - 据我了解，使用索引将无济于事，因为读取了大表中的所有行，并且读取了每个小表中的一半行，因为目前只有两种语言（DE、EN）。
- 一个糟糕的尝试是首先
UNION所有小表，因为它们都具有相同的列结构。然后我多次对结果进行LEFT JOIN。但没有改善。
有什么办法可以加快速度吗？喜欢parallel hash joins？这是如何运作的？或者是否可以设置其他参数？
【问题讨论】：
-
调优真实查询的第一步是查看真实的查询计划。
-
请edit您的问题并添加使用
explain (analyze, buffers, format text)生成的完整执行计划-不仅仅是您认为有趣的部分。最好打开track_io_timing。 -
您使用的是哪个 Postgres 版本？
-
我已经在问题中添加了真正的查询计划、每个节点类型的统计信息和 Postgres 版本
-
故障排除建议：试验一两张小桌子和大桌子。 Edit 你的问题，或者写另一个，向我们展示这些表的定义、索引和行数以及查询计划。目标：如果可能的话，在小表上建立更好的索引。观察：散列连接对于您对小表进行的那种名称查找来说是一种合理的策略。这就是你在应用程序中做同样事情的方式。
标签： sql postgresql left-join query-optimization