Comment by jihadjihad
12 hours ago
The section on multi-column indexes mirrors how I was taught and how I’ve generally handled such indexes in the past. But is it still true for more recent PG versions? I had an index and query similar to the third example, and IIRC PG was able to use an index, though I believe it was a bitmap index scan.
I am also unsure of the specific perf tradeoffs between index scan types in that case, but when I saw that happen in the EXPLAIN plan it was enough for me to call into question what had been hardcoded wisdom in my mind for quite some time.
Further essential reading is the classic Use The Index, Luke [0] site, and the book is a great buy for the whole team.
> The section on multi-column indexes mirrors how I was taught and how I’ve generally handled such indexes in the past. But is it still true for more recent PG versions?
No, it isn't. PostgreSQL 18 added support for index skip scan:
https://youtu.be/RTXeA5svapg?si=_6q3mj1sJL8oLEWC&t=1366
It's actually possible to use a multicolumn index with a query that only has operators on its lower-order columns in earlier versions. But that requires a full index scan, which is usually very inefficient.
Hi Peter, author here. Thanks for weighing in with the extra context on index skip scan, and huge thanks for adding this to Postgres.
I’m going to revise the multi-column index section to be more precise about when leftmost-prefix rules apply, and I’ll include a note on how skip scan changes the picture
A bitmap index scan allows the database to narrow down which pages could include the data, but then still has to recheck the condition on the contents of those pages - so will still not be as performant as an proper index scan
With postgres indexes not containing liveness data for tuples you'll have to hit quite a lot of those pages anyway, unless they are frozen.