In the plan below, we can see that the optimizer is sorting an already
sorted result. It seems to forget the sort order across the UNIQUE node. My
question is, do we make any attempts in the optimizer to remember the sort
order of a result, to avoid any further sorting on same sort-key? If not,
can we do something about it?
postgres=# explain select * from del where ctid in ( select ('''(' || i ||
',' || j || ')''')::tid from generate_series( 0, 1) s1(i), generate_series(
1, 1 ) s2(j) );
QUERY
PLAN
Merge Join (cost=177447.07..182043.29 rows=4 width=97)
Merge Cond: ((('''('::text || (s1.i)::text) || ','::text) ||
(s2.j)::text) || ')'''::text))::tid) = del.ctid)
- Sort (cost=155639.89..155739.89 rows=4 width=8)
Sort Key: (('''('::text || (s1.i)::text) || ','::text) ||
(s2.j)::text) || ')'''::text))::tid)
- Unique (cost=147032.84..152032.84 rows=4 width=8)
- Sort (cost=147032.84..149532.84 rows=100 width=8)
Sort Key: (('''('::text || (s1.i)::text) ||
','::text) || (s2.j)::text) || ')'''::text))::tid)
- Nested Loop (cost=13.50..20026.00 rows=100
width=8)
- Function Scan on generate_series s1
(cost=0.00..12.50 rows=1000 width=4)
- Materialize (cost=13.50..23.50 rows=1000
width=4)
- Function Scan on generate_series s2
(cost=0.00..12.50 rows=1000 width=4)
- Materialize (cost=21807.19..23055.61 rows=99874 width=103)
- Sort (cost=21807.19..22056.87 rows=99874 width=103)
Sort Key: del.ctid
- Seq Scan on del (cost=0.00..2586.74 rows=99874
width=103)
(15 rows)
Best regards,
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