Message-Id: <3.0.5.32.20000717141152.022dd100@mail.rhyme.com.au>
Date: Mon, 17 Jul 2000 14:11:52 +1000
To: Carolyn Lu Wong <carolyn@kss.net.au>
From: Philip Warner <pjw@rhyme.com.au>
Subject: Re: SQL question
Cc: "pgsql-sql@postgresql.org" <pgsql-sql@postgresql.org>
In-Reply-To: <39728072.31520F2F@kss.net.au>
References: <3.0.5.32.20000717121856.022cd100@mail.rhyme.com.au>
Mime-Version: 1.0
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At 13:41 17/07/00 +1000, Carolyn Lu Wong wrote:
>> Try
>>     select count(*) from table1 where account_no = 1 and start_date_time is
>> null;
>> 
>> and see if you get 0.
>
>Yes, i get 0 from running the above query, but it fails if i re-arrange
>the where clause to:
>
>	select * from table1
>	where start_date_time::date >= '01/01/2000'::date
>	and start_date_time::date <= '01/01/2001'::date
>	and account_no = 1;
>
>with the same error message.
>

I think that there is no guarantee of the order of evaluation of the
components of a predicate, but that putting 'account_no=1' early in the
statement means the row is excluded before it needs to evaluate the rest of
the statement. I don't think you should rely on this behaviour - it might
be classified as "it's a feature, not a bug". Maybe.

You probably need to tell me what NULLs in the start_date mean. If, eg,
they mean 'not started', then you could create a view:

    create view started_things as select * from table1 where not start_date
is null;

then use:

    	select * from started_things
 		where start_date_time::date >= '01/01/2000'::date
		and start_date_time::date <= '01/01/2001'::date
		and account_no = 1;

AFAICT, this will still use nice indexes etc, but I could be wrong.

Another alternative would be to define a 'coalesce' function (I don't think
PG has one), which takes an arbitrary number of arguments and returns the
first non-null one. You could then say "where coalesce(start_date_time,
'1/1/1500')::date >= '01/01/2000'::date" etc, but then I think you will
lose the effectiveness of indexes.

Maybe someone else has a better idea...


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