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Re: force partition pruning - Mailing list pgsql-general

From	Vijaykumar Jain
Subject	Re: force partition pruning
Date	May 11, 2021 14:51:03
Msg-id	CAM+6J96R9i2zU0CrvHSwK+0LdAMeW=kQ0VnznRxh_9h-NO3tjg@mail.gmail.com Whole thread Raw
In response to	Re: force partition pruning (Vijaykumar Jain <vijaykumarjain.github@gmail.com>)
Responses	SV: force partition pruning
List	pgsql-general

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Sorry,

I made a major mistake. I somehow saw the period and period_version as the same.

so, yes partitions are not pruned here. So my suggestion makes no sense.

***********************

create table register(period_version text) partition by list ( period_version );

create table register_p1 partition of register for values in ('201712_1');

create table register_p2 partition of register for values in ('201812_1');

create table register_p3 partition of register for values in ('201912_1');

insert into register values ('201712_1'), ('201812_1'), ('201912_1');

create table register_metadata( period_version text, period text );

insert into register_metadata values ('201912_1', '201912' );

insert into register_metadata values ('201812_1', '201812' );

insert into register_metadata values ('201712_1', '201712' );

postgres=# select * from register_metadata m where m.period between '201512' and '201712';

period_version | period

----------------+--------

201712_1 | 201712

(1 row)

postgres=# explain analyze select * from register where period_version in ('201712_1');

QUERY PLAN

----------------------------------------------------------------------------------------------------------------

Seq Scan on register_p1 register (cost=0.00..27.00 rows=7 width=32) (actual time=0.006..0.007 rows=1 loops=1)

Filter: (period_version = '201712_1'::text)

Planning Time: 0.192 ms

Execution Time: 0.016 ms

(4 rows)

explain analyze select * from register d join register_metadata m using (period_version) where m.period between '201512' and '201712'; -- partition not pruned

QUERY PLAN

---------------------------------------------------------------------------------------------------------------------------

Hash Join (cost=23.25..130.57 rows=82 width=64) (actual time=0.026..0.032 rows=1 loops=1)

Hash Cond: (d.period_version = m.period_version)

-> Append (cost=0.00..91.20 rows=4080 width=32) (actual time=0.004..0.009 rows=3 loops=1)

-> Seq Scan on register_p1 d_1 (cost=0.00..23.60 rows=1360 width=32) (actual time=0.004..0.004 rows=1 loops=1)

-> Seq Scan on register_p2 d_2 (cost=0.00..23.60 rows=1360 width=32) (actual time=0.002..0.002 rows=1 loops=1)

-> Seq Scan on register_p3 d_3 (cost=0.00..23.60 rows=1360 width=32) (actual time=0.001..0.001 rows=1 loops=1)

-> Hash (cost=23.20..23.20 rows=4 width=64) (actual time=0.011..0.012 rows=1 loops=1)

Buckets: 1024 Batches: 1 Memory Usage: 9kB

-> Seq Scan on register_metadata m (cost=0.00..23.20 rows=4 width=64) (actual time=0.007..0.008 rows=1 loops=1)

Filter: ((period >= '201512'::text) AND (period <= '201712'::text))

Rows Removed by Filter: 2

Planning Time: 0.208 ms

Execution Time: 0.051 ms

***********************

from the docs,

PostgreSQL: Documentation: 13: 5.11. Table Partitioning

During initialization of the query plan. Partition pruning can be performed here for parameter values which are known during the initialization phase of execution. Partitions which are pruned during this stage will not show up in the query's EXPLAIN or EXPLAIN ANALYZE. It is possible to determine the number of partitions which were removed during this phase by observing the “Subplans Removed” property in the EXPLAIN output.
During actual execution of the query plan. Partition pruning may also be performed here to remove partitions using values which are only known during actual query execution. This includes values from subqueries and values from execution-time parameters such as those from parameterized nested loop joins. Since the value of these parameters may change many times during the execution of the query, partition pruning is performed whenever one of the execution parameters being used by partition pruning changes. Determining if partitions were pruned during this phase requires careful inspection of the loops property in the EXPLAIN ANALYZE output. Subplans corresponding to different partitions may have different values for it depending on how many times each of them was pruned during execution. Some may be shown as (never executed) if they were pruned every time.

but from the tests in the source

postgres/partition_prune.sql at master · postgres/postgres (github.com)

if we make a little change to suit your case

-- Join

create table tbl1(col1 int);

insert into tbl1 values (501), (505);

-- Basic table

create table tprt (col1 int) partition by range (col1);

create table tprt_1 partition of tprt for values from (1) to (501);

create table tprt_2 partition of tprt for values from (501) to (1001);

create table tprt_3 partition of tprt for values from (1001) to (2001);

create table tprt_4 partition of tprt for values from (2001) to (3001);

create table tprt_5 partition of tprt for values from (3001) to (4001);

create table tprt_6 partition of tprt for values from (4001) to (5001);

create index tprt1_idx on tprt_1 (col1);

create index tprt2_idx on tprt_2 (col1);

create index tprt3_idx on tprt_3 (col1);

create index tprt4_idx on tprt_4 (col1);

create index tprt5_idx on tprt_5 (col1);

create index tprt6_idx on tprt_6 (col1);

insert into tprt values (10), (20), (501), (502), (505), (1001), (4500);

-- make this similar to your query.

alter table tbl1 add column col2 int default 0;

update tbl1 set col2 =1 where col1 = 501;

table tbl1;

col1 | col2

------+------

501 | 1

505 | 0

explain analyze select * from tprt where tprt.col1 in (select tbl1.col1 from tbl1 where tbl1.col2 in (1, 2) );

partition pruning does take place.

explain analyze select * from tprt where tprt.col1 in (select tbl1.col1 from tbl1 where tbl1.col2 in (1, 2) );

QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------

Nested Loop (cost=38.46..382.58 rows=1760 width=4) (actual time=0.026..0.029 rows=1 loops=1)

-> HashAggregate (cost=38.31..38.53 rows=22 width=4) (actual time=0.012..0.013 rows=1 loops=1)

Group Key: tbl1.col1

Batches: 1 Memory Usage: 24kB

-> Seq Scan on tbl1 (cost=0.00..38.25 rows=23 width=4) (actual time=0.008..0.009 rows=1 loops=1)

Filter: (col2 = ANY ('{1,2}'::integer[]))

Rows Removed by Filter: 1

-> Append (cost=0.15..14.86 rows=78 width=4) (actual time=0.011..0.013 rows=1 loops=1)

-> Index Only Scan using tprt1_idx on tprt_1 (cost=0.15..2.47 rows=13 width=4) (never executed)