Re: Query with concatenation in explain plan
Date: Sat, 29 Nov 2008 13:49:25 +0100
> I have a customer who has a bad-performing query, and the explain plan
> shows a split in the query with a concatenation at the end. Because the
> query is fairly complex, the two parts of the query take a lot of time,
> where (to my opinion) the whole part could be done in one run.
> I stripped the query to the point where the concatenation comes in. In
> the real query, it is joined with a lot of extra tables, of which some
> are full table scans (still have to tune that part).
> This is what it boils down to this (the query is generated by a software
> package, so I can't change them without changing the code of the package):
> SELECT mtb.*
> FROM MYTABLE mtb
> WHERE mtb.dt_begin between nvl(:b14,mtb.dt_begin) and
> SELECT STATEMENT ALL_ROWS
> Cost: 748 Bytes: 275,550 Cardinality: 2,505
> 6 CONCATENATION
> 2 FILTER
> 1 TABLE ACCESS FULL TABLE APP.MYTABLE
> Cost: 727 Bytes: 250,470 Cardinality: 2,277
> 5 FILTER
> 4 TABLE ACCESS BY INDEX ROWID TABLE APP.MYTABLE Cost: 22 Bytes:
> 25,080 Cardinality: 228
> 3 INDEX RANGE SCAN INDEX APP.mtb_IX_DT_BEGIN
> Cost: 3 Cardinality: 41
> Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit
> PL/SQL Release 10.2.0.4.0 - Production
> CORE 10.2.0.4.0 Production
> TNS for IBM/AIX RISC System/6000: Version 10.2.0.4.0 - Production
> NLSRTL Version 10.2.0.4.0 - Production
> Looks like a QBE where a user can put in date limits. If nothing is
> entered, all rows are returned.
> I noted that the problem is in the NVL's. A normal 'between' would use
> the index in one run. But still: a concatenation suggests that rows that
> are returned in the first clause and rows that are returned in the
> second are put together in the result set, where I would think only rows
> that are returned by BOTH clauses should be i the result set.
> Fron the docs: Concatenation = An operation that accepts multiple sets
> of rows and returns the union-all of the sets.
> Am I missing something here?
Right, I did miss something here... FILTER!
If I strip this to:
FROM mytable mtc
WHERE mtb.dt_begin <= nvl(:b14,mtb.dt_begin)
I still get the concatenation:
SELECT STATEMENT ALL_ROWS
Cost: 1,094 Bytes: 5,511,220 Cardinality: 50,102 6 CONCATENATION
1 TABLE ACCESS FULL TABLE APP.MYTABLE Cost: 727 Bytes: 5,010,170 Cardinality: 45,547 5 FILTER 4 TABLE ACCESS BY INDEX ROWID TABLE APP.MYTABLE Cost: 366 Bytes: 501,050 Cardinality: 4,555 3 INDEX RANGE SCAN INDEX APP.MTB_IX_DT_BEGIN Cost: 3 Cardinality: 820
It's not a concatenation between <= and >= but between null and not null of :b14.
What is strange and misleading though is that the costs and cardinality of both parts add up; where it should be either the first or the second....
Shakespeare Received on Sat Nov 29 2008 - 06:49:25 CST