Frequently one wants a data set returned in the same order as the
index used in the query. Informix (at least) has implicit order-by,
which means that the data will be returned in collating order if the
query forces use of the appropriate index.

Does Postgresql do this?

If not, does an Order-by force a sort even if an index has the correct
order to satisfy the order-by?

Thanks

John Moore
newsNOSPAM@tinyvital.com



Fri, 08 Jun 2001 16:49:13 GMT
"earthlink" <NOSPAMnews@tinyvital.com>
Implicit order-by in Postgresql?
earthlink wrote:
> 
> Frequently one wants a data set returned in the same order as the
> index used in the query. Informix (at least) has implicit order-by,
> which means that the data will be returned in collating order if the
> query forces use of the appropriate index.
> 
> Does Postgresql do this?

If data is fetched by walking an index, then , yes it produces an 
implicit order. Just don't count on it to stay that way - the use of a 
particular index depends on too many things.

> If not, does an Order-by force a sort even if an index has the correct
> order to satisfy the order-by?

Dropping unneeded sort node from a query tree was one of the earliest 
optimisations in postgresql.

---------------
Hannu

Tue, 12 Jun 2001 22:50:06 +0500
Hannu Krosing <hannu@tm.ee>
Re: Implicit order-by in Postgresql?