Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?
Date: Tue, 4 Mar 2003 13:17:13 -0000
"Lauri Pietarinen" <lauri.pietarinen_at_atbusiness.com> wrote in message
> Paul Vernon wrote:
> >FYI. DB2 does do some optimisation (I.e. does not join to S), but does
> not go
> >the whole hog in it's rewritten SQL (see below)
> >However it does set it's 'Early Out Flag' in the access plan on the join
> >between P and SP, which would AKAIK have the same effect as the EXISTS
> >in the 'more fully' optimised SQL suggest above.
> I agree. I think that "early out" flag means that it will stop as soon
> as it has found
> one qualifying SP-row, so in essence it equates to the EXISTS-version.
> Also the missing SORT implies that it does not have to eliminate duplicates.
> But why does it do a HASH-join?
Pass, but it's interesting to note that given the EXISTS form of the query,
DB2 optimises it to the same query as before.
SELECT P#, PNAME
WHERE P.P# = SP.P#) Optimized Statement:
SELECT DISTINCT Q2.P# AS "P#", Q2.PNAME AS "PNAME" FROM DB2.SP AS Q1, DB2.P AS Q2
WHERE (Q2.P# = Q1.P#) Access Plan:
Total Cost: 25.338 Query Degree: 1 Rows RETURN ( 1) Cost I/O | 88 HSJOIN ( 2) 25.338 1 /------+-----\ 88 137 TBSCAN IXSCAN ( 3) ( 4) 25.1328 0.182339 1 0 | | 88 137 TABLE: DB2 INDEX: SYSIBM P SQL0303031750371
Business Intelligence, IBM Global Services Received on Tue Mar 04 2003 - 14:17:13 CET