> On Mar 23, 2025, at 20:42, Marcelo Fernandes <marcefe...@gmail.com> wrote:
> Cons:
> 1. Sequential Scan
> 2. If the table is bloated, it reads more buffers.
These concerns probably sound worse than they really are. Assuming the table
is being vacuumed reliably, and is receiving inserts, those inserts will tend
to be at the start of the table, and so you'll hit a live tuple pretty fast.
There are pathological cases where it will take a long time (someone just
delete a DELETE FROM t; from a huge table and the entire thing is dead tuples
but no free space), but that's relatively unlikely to happen in a real-world
system.
> Pros:
> 1. Does an index-only scan on a field that presumably has a PK index.
> 2. Works well even if the table is bloated.
#1 is not necessarily true. You're never guaranteed an index-only scan;
PostgreSQL may decide that the visibility map information indicates that a
regular index scan (which means heap fetches to check visibility) will still
happen.
#2 is not necessarily true either; it's pretty much false in the same situation
that a sequential scan for the first tuple will be slow. Indexes get bloated,
too, and if there are a large number of dead unreclaimed tuples, there are also
going to be a lot of dead index entries for it to walk over.
(And to be pedantic, #1 returns TRUE / FALSE while #2 returns <int> / NULL, so
they aren't exactly equivalent. If you are able to handle <int> / NULL, you
don't need the EXISTS clause.)
> SELECT count(*) FROM foo;
There's no situation in which this will be faster than #1 or #2.
