Re: Performance improvement for joins where outer side is unique - Mailing list pgsql-hackers

On 4 September 2015 at 04:50, Robert Haas <robertmhaas@gmail.com> wrote:

Also: very nice performance results.

Thanks.

On following a thread in [General] [1] it occurred to me that this patch can give a massive improvement on Merge joins where the mark and restore causes an index scan to have to skip over many filtered rows again and again.

I mocked up some tables and some data from the scenario on the [General] thread:

create table customers (id bigint, group_id bigint not null);

insert into customers select x.x,x.x%27724+1 from generate_series(1,473733) x(x);

alter table customers add constraint customer_pkey primary key (id);

create table balances (id bigint, balance int not null, tracking_number int not null, customer_id bigint not null);

insert into balances select x.x, 100, 12345, x.x % 45 + 1 from generate_Series(1,16876) x(x);

create index balance_customer_id_index on balances (customer_id);

create index balances_customer_tracking_number_index on balances (customer_id,tracking_number);

analyze;

Unpatched I get:

test=# explain analyze SELECT ac.* FROM balances ac join customers o ON o.id = ac.customer_id WHERE o.group_id = 45;

                                                                       QUERY PLAN

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

 Merge Join  (cost=164.87..1868.70 rows=1 width=24) (actual time=6.110..1491.408 rows=375 loops=1)

   Merge Cond: (ac.customer_id = o.id)

   ->  Index Scan using balance_customer_id_index on balances ac  (cost=0.29..881.24 rows=16876 width=24) (actual time=0.009..5.206 rows=16876 loops=1)

   ->  Index Scan using customer_pkey on customers o  (cost=0.42..16062.75 rows=17 width=8) (actual time=0.014..1484.382 rows=376 loops=1)

         Filter: (group_id = 45)

         Rows Removed by Filter: 10396168

 Planning time: 0.207 ms

 Execution time: 1491.469 ms

(8 rows)

Patched:

test=# explain analyze SELECT ac.* FROM balances ac join customers o ON o.id = ac.customer_id WHERE o.group_id = 45;

                                                                       QUERY PLAN

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

 Merge Join  (cost=164.87..1868.70 rows=1 width=24) (actual time=6.037..11.528 rows=375 loops=1)

   Merge Cond: (ac.customer_id = o.id)

   ->  Index Scan using balance_customer_id_index on balances ac  (cost=0.29..881.24 rows=16876 width=24) (actual time=0.009..4.978 rows=16876 loops=1)

   ->  Index Scan using customer_pkey on customers o  (cost=0.42..16062.75 rows=17 width=8) (actual time=0.015..5.141 rows=2 loops=1)

         Filter: (group_id = 45)

         Rows Removed by Filter: 27766

 Planning time: 0.204 ms

 Execution time: 11.575 ms

(8 rows)

Now it could well be that the merge join costs need a bit more work to avoid a merge join in this case, but as it stands as of today, this is your performance gain.

Regards

David Rowley

[1] http://www.postgresql.org/message-id/CACZYdDiAxEAM2RkMHBMuhwVcM4txH+5E3HQGgGyzfzbn-PnvFg@mail.gmail.com

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