Re: extremly bad select performance on huge table

=?ISO-8859-15?Q?Bj=F6rn_Wittich?= <Bjoern_Wittich(at)gmx(dot)de> writes:
> Here is the explain (analyze,buffers) select mycolumn from myhugetable
> "Index Only Scan using myprimkey on myhugetable (cost=0.00..8224444.82
> rows=71768080 width=33) (actual time=16.722..2456300.778 rows=71825999
> loops=1)"
> " Heap Fetches: 356861"
> " Buffers: shared hit=71799472 read=613813"
> "Total runtime: 2503009.611 ms"

So that works out to about 4 msec per page fetched considering only I/O
costs, which is about as good as you're likely to get if the data is
sitting on spinning rust.

You could potentially make it faster with a VACUUM (to mark all pages
all-visible and eliminate the "heap fetches" costs), or a REINDEX
(so that the index scan becomes more nearly sequential instead of random
access). However, unless the data is nearly static those will just be
temporary fixes: the time will degrade again as you update the table.

> Note: This select is just for testing. My final statement will be a join
> on this table via the "mycolumn" column.

In that case it's probably a waste of time to worry about the performance
of this query as such. In the first place, a join is not likely to use
the index at all unless it's fetching a relatively small number of rows,
and in the second place it seems unlikely that the join query can use
an IndexOnlyScan on this index --- I imagine that the purpose of the join
will require fetching additional columns.

regards, tom lane