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这篇文章主要讲解了“怎么解决 PostgreSQL 窗口函数调用的限制”，文中的讲解内容简单清晰，易于学习与理解，下面请大家跟着丸趣 TV 小编的思路慢慢深入，一起来研究和学习“怎么解决 PostgreSQL 窗口函数调用的限制”吧！
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)感谢各位的阅读，以上就是“怎么解决 PostgreSQL 窗口函数调用的限制”的内容了，经过本文的学习后，相信大家对怎么解决 PostgreSQL 窗口函数调用的限制这一问题有了更深刻的体会，具体使用情况还需要大家实践验证。这里是丸趣 TV，丸趣 TV 小编将为大家推送更多相关知识点的文章，欢迎关注！
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