RE: Question on how cardinality is calculated.
Date: Sat, 12 Jan 2008 13:12:06 -0500
One more thing - somewhere else in the thread you've asserted there is exactly one match for the case like '%123456%'.
Now without reengineering the data you have, you're going to have to at least scan an index on case. If I recall correctly you've got something like 1.3 M rows in enquiry and .11 M rows in cases, and currently you get what you need in about 5 seconds (deemed unacceptable). If about .5 seconds is acceptable, then if you don't have any type conversions going on it seems likely the optimizer will choose either a table scan or a fast full index scan on the case index to get you cases.id and then use the index lookup on enquiry.id if you formulate the query as:
select c.a, c.b, c.c, c.case, e.a, e.b
from enquiry e, cases c
where e.id =
( select c2.id from cases c2 where c2.case like '%123456%' and rownum < 2 ) and e.id = c.id;
You can leave out the "and rownum < 2" bit if you want it to blow up if your assertion is wrong. You could also switch it to an "in" clause, but I haven't tested whether the optimizer uses a presumptive 1 cardinality for equals when the syntax will otherwise blow up. (It should, since any other cardinality should blow up.) For an "in" clause the optimizer could not validly make that assumption. Without testing your exact case in your exact environment, I don't know enough to predict whether either the "=" or "in" alternative will have its predicates pushed so you end up with still scanning enquiry for a hash resolution. Notice there is a presumption that c.id is unique - if you had multiple different cases for a single id this would not be an isofunctional rewrite.
Jonathan's suggestion to reengineer the data collection should result in a very small difference between actual fetch time and zero, since the optimizer per that suggestion should be able to index lookup a single row from cases and the one or two rows by index for enquiry. But if you can't do that, the re-write above might work out good enough.
From: Mark W. Farnham [mailto:mwf_at_rsiz.com] Sent: Thursday, January 10, 2008 3:45 PM To: 'ronnie_doggart_at_lagan.com'; 'jonathan_at_jlcomp.demon.co.uk'; 'oracle-l_at_freelists.org'
Subject: RE: Question on how cardinality is calculated.
select c.a, c.b, c.c, c.case, e.a, e.b
from enquiry e,
(select id, a, b, c, case from cases where case like '%123456%) c where e.id = c.id;
notice I've eliminated the upper function, since unless there is some character set I am ignorant of where digits are stored differently for upper and lower case, it is irrelevant.
if that does not immediately work, you might embark on hinting. Presumably you want to hint either the index on case for the virtual table query or cardinality of 1 and hint the id index on enquiry. Adding the pseudocolumn rownum to the virtual table query might also help, like:
select c.crownum,c.a,c.b,c.c,c.case, e.a, e.b
from enquiry e,
(select rownum crownum,id, a, b, c, case
from cases where case like '%123456%') c
where e.id = c.id;
if the cardinality hint does not function correctly (if I recall correctly that comes and goes at various releases and I'm not sure which) and the hint to use the case index does not generate the lowest cost join plan because of the 5% thing, then you might get the case rows through a gtt, although that does not in general scale and grabs latches you probably don't want to grab if this is a busy system in a parallel sense. If it is not high concurrency and you just need snappier response for a few users, at least the optimizer will have good cardinality in hand from the gtt subset of case so it should use the index on enquiry. Notice this is definitely not your first choice. If you're already slapping a generated literal in there for the "case like" to reparse it probably scales no worse than what you're currently doing.
You might think that further adding a "and rownum < 2" predicate to the virtural table query would stand in on releases where the cardinality hint does not function as expected, but my testing has not found a release on which Oracle semantically scans for a rownum predicate to definitely limit the cardinality. They could reliably do this, but for whatever reason they apparently do not. Of course adding that substitutes your assurance that only one row can be retrieved for real world results, but I have not experienced it working, anyway.
JL can probably suggest a more direct way to get the plan you want.
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Ronnie Doggart
Sent: Thursday, January 10, 2008 12:33 PM To: jonathan_at_jlcomp.demon.co.uk; oracle-l_at_freelists.org Subject: RE: Question on how cardinality is calculated.
Thanks to those who answered, the majority pointing out that it is 5% of the number of rows.
The oracle version is 126.96.36.199
To build on the information provided, the actual query used in an application does the following:
select cases.a, cases.b, cases.c, cases.case, enquiry.a, enquiry.b,
from cases, enquiry
enquiry.id = cases.id
and upper(case) like '%123456%';
Now the user always enters six digits which I know identifies 1 record in the cases table, but because Oracle uses a cardinality of 5553 for this it performs a hash join to the enquiry table, even though only one record will match in the enquiry table. The enquiry table contains 1.4 million record so it takes ~5 seconds.
Would it be advisable to put a hint on the query to either use a nested loop which allows the query to use an index to retrieve the data from enquiry table. Or use a cardinality hint since I know we will not be returning 5553 rows which will also allow the use of an index.
From: oracle-l-bounce_at_freelists.org on behalf of Jonathan Lewis
Sent: Thu 1/10/2008 4:22 PM
To: Ronnie Doggart; oracle-l_at_freelists.org Subject: Re: Question on how cardinality is calculated.
5% of num_rows
Author: Cost Based Oracle: Fundamentals
The Co-operative Oracle Users' FAQ
- Original Message ----- From: "Ronnie Doggart" <ronnie_doggart_at_lagan.com> To: <oracle-l_at_freelists.org> Sent: Thursday, January 10, 2008 3:06 PM Subject: Question on how cardinality is calculated.
Does anyone know how Oracle calculates the cardinality of a query such as:
Select * from table_1 where upper(case) like '%12345%';
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=131 Card=5553 Bytes=621936)
1 0 TABLE ACCESS (FULL) OF 'TABLE_1' (Cost=131 Card=5553 Bytes=621936)
Oracle calculates that it is expecting 5553 rows from this query but I know it will only return 2.
How does it do its estimate ?
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