On Wed, 01 Mar 2017 11:12:29 -0500, Tom Lane <tgl@sss.pgh.pa.us>
wrote:
>This is a great example of "select distinct" being used as a band-aid
>over a fundamental misunderstanding of SQL. It's good advice to never use
>"distinct" unless you know exactly why your query is generating duplicate
>rows in the first place.
On that note:
I know most people here don't pay much - or any - attention to
SQLServer, however there was an interesting article recently regarding
significant performance differences between DISTINCT and GROUP BY as
used to remove duplicates.
https://sqlperformance.com/2017/01/t-sql-queries/surprises-assumptions-group-by-distinct
Now I'm wondering if something similar might be lurking in Postgresql?
[Yeah, I know - test it and find out!
Thing is, the queries used in the article are not simple. Although
not explicitly stated, it hints that - at least for SQLServer - a
simple case involving a string column is probably insufficient, and
complex scenarios are required to produce significant differences.
]
I'll get around to doing some testing soon. For now, I am just asking
if anyone has ever run into something like this?
George
