4 ms·
Count on an indexed column is a pretty fast operation and it's always correct.
by Pharaoh2 11y ago
Count on an indexed column is a pretty fast operation and it's always correct.
- calpaterson 11y agocount(*) is a linear time operation (roughly to the size of the table) on postgres. It does an index-only scan but with a sufficiently large table/index it's still a problem. It's to do with postgres MVCC implementation. I don't know if other SQL databases have this problem. MySQL doesn't seem to.
- anarazel 11y agoIt's a problem in mysql too, unless you use an engine like myisam, which isn't realistic in most scenarios due to durability and concurrency concerns.