怎么解决PostgreSQL窗口函数调用的限制
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背景
窗口函数是分析场景常用的，目前(citus 7.5)仅支持两种场景使用window函数，
1、partition by 必须是分布键。
2、where条件里面带分布键的等值过滤条件。
本质上：目前（citus 7.5）window函数不支持跨shard操作，或者说过程中不进行重分布。
而Greenplum这方面做得很好，是一个完整的MPP数据库。
citus window函数的支持
postgres=# \set VERBOSITY verbose postgres=# select row_number() over(partition by bid order by aid) rn,* from pgbench_accounts; ERROR: 0A000: could not run distributed query because the window function that is used cannot be pushed down HINT: Window functions are supported in two ways. Either add an equality filter on the distributed tables' partition column or use the window functions with a PARTITION BY clause containing the distribution column LOCATION: DeferErrorIfQueryNotSupported, multi_logical_planner.c:938
满足以下条件即可支持
1、partition by 必须是分布键。
2、where条件里面带分布键的等值过滤条件。
postgres=# select row_number() over(partition by bid order by aid) rn,* from pgbench_accounts where aid=1; rn | aid | bid | abalance | filler ----+-----+-----+----------+-------------------------------------------------------------------------------------- 1 | 1 | 1 | 0 | (1 row) postgres=# select row_number() over(partition by aid order by bid) rn,* from pgbench_accounts limit 1; rn | aid | bid | abalance | filler ----+-----+-----+----------+-------------------------------------------------------------------------------------- 1 | 298 | 1 | 0 | (1 row)
执行计划
postgres=# explain verbose select row_number() over(partition by aid order by bid) rn,* from pgbench_accounts limit 1; QUERY PLAN --------------------------------------------------------------------------------------------------------------------------------------------------------- Limit (cost=0.00..0.00 rows=0 width=0) Output: remote_scan.rn, remote_scan.aid, remote_scan.bid, remote_scan.abalance, remote_scan.filler -> Custom Scan (Citus Real-Time) (cost=0.00..0.00 rows=0 width=0) Output: remote_scan.rn, remote_scan.aid, remote_scan.bid, remote_scan.abalance, remote_scan.filler Task Count: 128 Tasks Shown: One of 128 -> Task Node: host=172.24.211.224 port=1921 dbname=postgres -> Limit (cost=705.99..706.01 rows=1 width=105) Output: (row_number() OVER (?)), pgbench_accounts.aid, pgbench_accounts.bid, pgbench_accounts.abalance, pgbench_accounts.filler -> WindowAgg (cost=705.99..860.95 rows=7748 width=105) Output: row_number() OVER (?), pgbench_accounts.aid, pgbench_accounts.bid, pgbench_accounts.abalance, pgbench_accounts.filler -> Sort (cost=705.99..725.36 rows=7748 width=97) Output: pgbench_accounts.aid, pgbench_accounts.bid, pgbench_accounts.abalance, pgbench_accounts.filler Sort Key: pgbench_accounts.aid, pgbench_accounts.bid -> Seq Scan on public.pgbench_accounts_106812 pgbench_accounts (cost=0.00..205.48 rows=7748 width=97) Output: pgbench_accounts.aid, pgbench_accounts.bid, pgbench_accounts.abalance, pgbench_accounts.filler (17 rows) postgres=# explain verbose select row_number() over(partition by bid order by aid) rn,* from pgbench_accounts where aid=1; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------------------------------- Custom Scan (Citus Router) (cost=0.00..0.00 rows=0 width=0) Output: remote_scan.rn, remote_scan.aid, remote_scan.bid, remote_scan.abalance, remote_scan.filler Task Count: 1 Tasks Shown: All -> Task Node: host=172.24.211.232 port=1921 dbname=postgres -> WindowAgg (cost=2.51..2.53 rows=1 width=105) Output: row_number() OVER (?), aid, bid, abalance, filler -> Sort (cost=2.51..2.51 rows=1 width=97) Output: aid, bid, abalance, filler Sort Key: pgbench_accounts.bid -> Index Scan using pgbench_accounts_pkey_106819 on public.pgbench_accounts_106819 pgbench_accounts (cost=0.28..2.50 rows=1 width=97) Output: aid, bid, abalance, filler Index Cond: (pgbench_accounts.aid = 1) (14 rows)
Citus未在window调用中支持重分布的过程。
greenplum window函数的支持
支持任意姿势的window调用
postgres=# create table t(id int, c1 int, c2 int); NOTICE: Table doesn't have 'DISTRIBUTED BY' clause -- Using column named 'id' as the Greenplum Database data distribution key for this table. HINT: The 'DISTRIBUTED BY' clause determines the distribution of data. Make sure column(s) chosen are the optimal data distribution key to minimize skew. CREATE TABLE postgres=# insert into t select random()*100000, random()*10, random()*100 from generate_series(1,10000000); INSERT 0 10000000 postgres=# explain select row_number() over (partition by c1 order by id) rn,* from t ; QUERY PLAN ------------------------------------------------------------------------------------------------------------------ Gather Motion 33:1 (slice2; segments: 33) (cost=1477974.88..1553064.94 rows=10012008 width=12) -> Window (cost=1477974.88..1553064.94 rows=303395 width=12) Partition By: c1 Order By: id -> Sort (cost=1477974.88..1503004.90 rows=303395 width=12) Sort Key: c1, id // 以下在citus中用临时表代替 -> redistribute Motion 33:33 (slice1; segments: 33) (cost=0.00..313817.24 rows=303395 width=12) Hash Key: c1 -> Seq Scan on t (cost=0.00..113577.08 rows=303395 width=12) Optimizer status: legacy query optimizer (10 rows)
甚至一个SQL中支持多个不同维度的partition
postgres=# explain select row_number() over (partition by c1 order by id) rn1, row_number() over (partition by c2 order by c1) rn2, * from t ; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------------------ Gather Motion 33:1 (slice3; segments: 33) (cost=3017582.83..3192792.97 rows=10012008 width=12) -> Subquery Scan coplan (cost=3017582.83..3192792.97 rows=303395 width=12) -> Window (cost=3017582.83..3092672.89 rows=303395 width=12) Partition By: coplan.c1 Order By: coplan.id -> Sort (cost=3017582.83..3042612.85 rows=303395 width=12) Sort Key: coplan.c1, coplan.id // 以下在citus中用临时表代替 -> Redistribute Motion 33:33 (slice2; segments: 33) (cost=1477974.88..1853425.18 rows=303395 width=12) Hash Key: coplan.c1 -> Subquery Scan coplan (cost=1477974.88..1653185.02 rows=303395 width=12) -> Window (cost=1477974.88..1553064.94 rows=303395 width=12) Partition By: t.c2 Order By: t.c1 -> Sort (cost=1477974.88..1503004.90 rows=303395 width=12) Sort Key: t.c2, t.c1 // 以下在citus中用临时表代替 -> Redistribute Motion 33:33 (slice1; segments: 33) (cost=0.00..313817.24 rows=303395 width=12) Hash Key: t.c2 -> Seq Scan on t (cost=0.00..113577.08 rows=303395 width=12) Optimizer status: legacy query optimizer (19 rows)
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