> On 14/02/2023 18:53 CET Sebastien Flaesch <sebastien.flaesch@4js.com> wrote:
>
> Hello!
>
> When creating an index on multiple columns, does the order of the columns
> matter? (I guess so)
Maybe, depending on the queries.
> It's mostly for SELECT statements using a condition that include ALL
> columns of the index (pkey):
>
> SELECT * FROM art WHERE etb='L1' and code='ART345'
>
> I would naturally put the columns with the most various values first, and
>
> For example, if the "code" column contains thousands of various item ids
> like 'SXZ874', 'ERF345', ... while the "etb" column contains a dozen of
> values like "L1", "LT" and "BX".
>
> Which one is best?
>
> CREATE UNIQUE INDEX ix1 ON art (code, etb)
> or
> CREATE UNIQUE INDEX ix1 ON art (etb, code)
>
> (or its PRIMARY KEY equivalent)
It should not make any difference for the query above. It can make a
difference for queries that only filter by the second index column or use
inequality constraints on those columns.
> Does it depend on the type of index (Btree, GiST, etc) ?
>
> I could not find that information in the doc.
Yes, see the documentation on multicolumn indexes with details on how they
are used: https://www.postgresql.org/docs/current/indexes-multicolumn.html
But you're limited to btree anyway if you're only interested in unique
indexes.
--
Erik
