On Tue, Sep 15, 2020 at 10:57:04PM +1200, David Rowley wrote: > On Fri, 31 Jul 2020 at 20:41, Pierre Ducroquet <p.p...@pinaraf.info> wrote: > > > > In a recent audit, I noticed that application developers have a tendency to > > abuse the distinct clause. For instance they use an ORM and add a distinct > > at > > the top level just because they don't know the cost it has, or they don't > > know > > that using EXISTS is a better way to express their queries than doing JOINs > > (or worse, they can't do better). > > > > They thus have this kind of queries (considering tbl1 has a PK of course): > > SELECT DISTINCT * FROM tbl1; > > SELECT DISTINCT * FROM tbl1 ORDER BY a; > > SELECT DISTINCT tbl1.* FROM tbl1 > > JOIN tbl2 ON tbl2.a = tbl1.id; > > This is a common anti-pattern that I used to see a couple of jobs ago. > What seemed to happen was that someone would modify some query or a > view to join in an additional table to fetch some information that was > now required. At some later time, there'd be a bug report to say that > the query is returning certain records more than once. The > developer's solution was to add DISTINCT, instead of figuring out that > the join that was previously added missed some column from the join > clause.
I can 100% imagine that happening. :-( -- Bruce Momjian <br...@momjian.us> https://momjian.us EnterpriseDB https://enterprisedb.com The usefulness of a cup is in its emptiness, Bruce Lee
