To: Marco Vezzoli <marco.vezzoli@st.com>
Cc: sql <pgsql-sql@postgresql.org>
Subject: Re: postgres 7.1.3: why does the query plan ignore indexes? 
In-reply-to: <3F41E343.AE3B5E@st.com> 
References: <3F41E343.AE3B5E@st.com>
Comments: In-reply-to Marco Vezzoli <marco.vezzoli@st.com>
	message dated "Tue, 19 Aug 2003 10:43:47 +0200"
Date: Tue, 19 Aug 2003 09:26:29 -0400
Message-ID: <27563.1061299589@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Marco Vezzoli <marco.vezzoli@st.com> writes:
> ewsweb_test=> \d measures_product
> Index "measures_product"
>  Attribute  |   Type   
> ------------+----------
>  product_id | smallint
                ^^^^^^^^

> ewsweb_test=> explain select zero_yield, gross from measures where
> product_id=29 and date between '2003-03-12' and '2003-08-14';
  ^^^^^^^^^^^^^

"29" is taken as an integer (int4).  To get an indexscan on an int2
column, you need to explicitly cast it to int2:
	product_id = 29::smallint
or you can put it in quotes and let the parser figure out the right
type:
	product_id = '29'

Yes, we'd like to make this better, but there are surprisingly many
pitfalls in tinkering with the assignment of datatypes to constants...

			regards, tom lane

PS: you could also consider whether it's really saving you any space to
store product_id as a smallint instead of int.  Because of alignment
considerations, it very possibly isn't.