Re: Multi-column index: Which column order - Mailing list pgsql-general

On Wed, 2023-02-15 at 10:20 -0600, Ron wrote:
> On 2/15/23 02:46, Laurenz Albe wrote:
> > 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)
> > Both are the same.
> >
> > There is an old myth that says that you should use the more selective column first
> > (which would be "code"), but that is just a myth.
>
> Only on Postgresql?

No, on all relational databases that use B-tree indexes.

Look at how many index entries have to be scanned in both cases, and you will see.

Yours,
Laurenz Albe