| From: | Jimmy A <jimmypsql(at)gmail(dot)com> |
|---|---|
| To: | pgsql-general(at)postgresql(dot)org |
| Subject: | Naked EXISTS vs SELECT EXISTS very different performance. |
| Date: | 2021-11-01 06:10:53 |
| Message-ID: | CANU97ysMJjzYUqo2COo7sZ=UcNrF1tHCbRAN40n3mMo-WSt7HQ@mail.gmail.com |
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| Lists: | pgsql-general |
Hi everyone.
Can someone tell me why these two equivalent queries, one involving a
"naked" EXISTS
versus one involving an EXISTS inside a SELECT statement perform so
differently?
I can see that the slow one scans the entire child table while the fast one
only scans children
that have the same parent_id as the parent. The tables have been vacuumed.
I have confirmed both give the same results across the whole table.
Setup is attached.
Cheers and regards.
| Attachment | Content-Type | Size |
|---|---|---|
| exists.sql | application/octet-stream | 1.2 KB |
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