On 19/04/2019 01:24, Ron wrote:
On 4/18/19 2:14 AM, Andreas Kretschmer wrote:
Am 18.04.19 um 08:52 schrieb rihad:
Hi. Say there are 2 indexes:
"foo_index" btree (foo_id)
"multi_index" btree (foo_id, approved, expires_at)
foo_id is an integer. Some queries involve all three columns in
their WHERE clauses, some involve only foo_id.
Would it be ok from general performance standpoint to remove
foo_index and rely only on multi_index? I know that
PG would have to do less work updating just one index compared to
updating them both, but wouldn't searches
on foo_id alone become slower?
it depends .
it depends on the queries you are using, on your workload. a
multi-column-index will be large than an index over just one column,
therefore you will have more disk-io when you read from such an index.
But two indexes are larger than one index, and updating two indexes
requires more disk IO than updating one index.
Agreed.
A key question would be: how often is the query run, compared to the
frequency Insertions, Updates, and Deletions -- wrt the table.
(Prefix compression would obviate the need for this question. Then
your multi-column index would be *much* smaller.)
True, but a multi column index will still be bigger than single column
index.
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