From: Bruce Momjian <pgman@candle.pha.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: 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?
Date: Tue, 26 Aug 2003 00:17:59 -0400 (EDT)
Message-ID: <200308260417.h7Q4Hxo01179@candle.pha.pa.us> (raw)
In-Reply-To: <27563.1061299589@sss.pgh.pa.us>
Should we consider adding some warning when someone creates an index on
an int2 column?
---------------------------------------------------------------------------
Tom Lane wrote:
> 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.
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: the planner will ignore your desire to choose an index scan if your
> joining column's datatypes do not match
>
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
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To: pgsql-sql@postgresql.org
Cc: pgman@candle.pha.pa.us, tgl@sss.pgh.pa.us, marco.vezzoli@st.com
Subject: Re: postgres 7.1.3: why does the query plan ignore indexes?
In-Reply-To: <200308260417.h7Q4Hxo01179@candle.pha.pa.us>
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