From: Josh Berkus <josh@agliodbs.com>
To: LORRAINE DEWEY <LORRAINE.DEWEY@companiongroup.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: Optomizing left outer joins
Date: Wed, 23 Apr 2003 13:51:53 -0700
Message-ID: <200304231351.53694.josh@agliodbs.com> (raw)
In-Reply-To: <NY20fb33-5d6a4829@companiongroup.com>
References: <NY20fb33-5d6a4829@companiongroup.com>
Lorraine,
> Perhaps I should have said that not only is the column optional, the entire
> row is optional. I am trying to join a master table (contains insurance
claim
> information) to an error table. If the claim in the master table doesn't
have
> any errors, there won't be a row for it in the error table. When I select
> columns from both tables, I only get the errored claims. That's why I was
> trying to use the outer join.
Well, an outer join is appropriate in that case. However, a single outer
join shouldn't be killing your queries -- in the example I gave, I eliminated
19 outer joins to boost the client's performance.
I think the lack of indexes is your real albatross.
> I like your advice about telling the customer about the workaround. I'll
> definitely do that! The workaround is pretty awkward: two select statements
> joined by "union all," then some subselects to pick out the unmatched
records.
> It returns the correct data and it's faster than the left join, but it
takes
> a lot more code. Anyway, I guess I'm stuck with it.
You probably are. But make sure you tell the client that the lack of indexes
is killing performance; even if it doesn't get fixed, your keister is
covered.
--
-Josh Berkus
Aglio Database Solutions
San Francisco
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-sql@postgresql.org
Cc: josh@agliodbs.com, LORRAINE.DEWEY@companiongroup.com
Subject: Re: Optomizing left outer joins
In-Reply-To: <200304231351.53694.josh@agliodbs.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox