So, uses the index:
explain analyze delete from statcast.play_statistics as ps
using stats as s
where ps.gid = s.gid
and ps.guid = s.guid;
It does this:
-> Index Scan using play_statistics_pkey on play_statistics ps (cost=0.69..8.59 rows=1 width=26) (actual time=0.008..0.065 rows=108 loops=21063)
Index Cond: ((gid = s.gid) AND (guid = s.guid))
Does not use the index:
explain analyze delete from statcast.play_statistics where gid in (select distinct gid from stats);
It does this:
-> Seq Scan on play_statistics (cost=0.00..7781818.16 rows=96600516 width=10) (actual time=201.521..396052.101 rows=96556116 loops=1)