RE: Index Contention / Sequence Caching
Date: Mon, 17 Mar 2014 22:48:53 +0000
Message-ID: <CE70217733273F49A8A162EE074F64D901DE37CD_at_exmbx05.thus.corp>
David,
If you look at your example you can see from the cost that the local optimizer is expecting to do a tablescan on the remote table (221 = 137 + 87 ... the 87 is NOT an estimated cost of doing a single indexed access for one row.)
The reason you get a nested loop join is because the local driver is expected to return a single row - which means the cost of the join for the nest loop is
cost of getting driving row + 1 * cost of getting related rows (tablescan). while the cost of the join for a hash join would be
cost of getting hash table + cost of getting hash table (tablescan) + cost of performance hash join
It's the classic case of "when the cardinality drops to 1 the next join may be a disaser".
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
_at_jloracle
From: oracle-l-bounce_at_freelists.org [oracle-l-bounce_at_freelists.org] on behalf of David Fitzjarrell [oratune_at_yahoo.com] Sent: 17 March 2014 16:49
To: riyaj.shamsudeen_at_gmail.com; Mohamed Houri Cc: suzzell; oracle-l_at_freelists.org
Subject: Re: Index Contention / Sequence Caching
The first example cited by Jonathan Lewis was for Oracle 8i; running the code on 11.2.0.3 I don't see that same behavior:
SQL> select
2 /*+ 3 leading(t2) use_nl(t1) 4 */ 5 t2.object_name, t1.object_name 6 from 7 t2 t2, 8 t1_at_poojooba t1 9 where 10 t2.object_name = 'DUAL'
11 and t1.object_id = t2.object_id
12 ;
OBJECT_NAME OBJECT_NAME ------------------------------ ------------------------------ DUAL DUAL DUAL DUAL
Execution Plan
Plan hash value: 3485226535
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|
| 0 | SELECT STATEMENT | | 1 | 54 | 221 (0)| 00:00:03 | | |
| 1 | NESTED LOOPS | | 1 | 54 | 221 (0)| 00:00:03 | | |
|* 2 | TABLE ACCESS FULL| T2 | 1 | 24 | 134 (0)| 00:00:02 | | |
| 3 | REMOTE | T1 | 1 | 30 | 87 (0)| 00:00:02 | POOJO~ | R->S |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
2 - filter("T2"."OBJECT_NAME"='DUAL')
Remote SQL Information (identified by operation id):
3 - SELECT /*+ USE_NL ("T1") */ "OBJECT_NAME","OBJECT_ID" FROM "T1" "T1" WHERE "OBJECT_ID"=:1 (accessing 'POOJOOBA' )
Statistics
1 recursive calls 0 db block gets 478 consistent gets 0 physical reads 0 redo size 670 bytes sent via SQL*Net to client 519 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed
SQL>
SQL> alter index t1_i1 rebuild reverse;
Index altered.
SQL>
SQL> select
2 /*+ 3 leading(t2) use_nl(t1) 4 */ 5 t2.object_name, t1.object_name 6 from 7 t2 t2, 8 t1_at_poojooba t1 9 where 10 t2.object_name = 'DUAL'
11 and t1.object_id = t2.object_id
12 ;
OBJECT_NAME OBJECT_NAME ------------------------------ ------------------------------ DUAL DUAL DUAL DUAL
Execution Plan
Plan hash value: 3485226535
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|
| 0 | SELECT STATEMENT | | 1 | 54 | 221 (0)| 00:00:03 | | |
| 1 | NESTED LOOPS | | 1 | 54 | 221 (0)| 00:00:03 | | |
|* 2 | TABLE ACCESS FULL| T2 | 1 | 24 | 134 (0)| 00:00:02 | | |
| 3 | REMOTE | T1 | 1 | 30 | 87 (0)| 00:00:02 | POOJO~ | R->S |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
2 - filter("T2"."OBJECT_NAME"='DUAL')
Remote SQL Information (identified by operation id):
3 - SELECT /*+ USE_NL ("T1") */ "OBJECT_NAME","OBJECT_ID" FROM "T1" "T1" WHERE "OBJECT_ID"=:1 (accessing 'POOJOOBA' )
Statistics
0 recursive calls 1 db block gets 478 consistent gets 0 physical reads 256 redo size 670 bytes sent via SQL*Net to client 519 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed
SQL> I do not disagree with the points raised in Jonathan's second post; those are valid concerns and should be considered carefully before implementing a reverse-key index.
David Fitzjarrell
Primary author, "Oracle Exadata Survival Guide"
On Monday, March 17, 2014 10:28 AM, Riyaj Shamsudeen <riyaj.shamsudeen_at_gmail.com> wrote:
Hello Stephen
In addition to Houri (and JL) has pointed out already, effect of a reverse key index is, to spread the values among ALL the leaf blocks of the index. So, the buffer cache(s) can be polluted with all these leaf blocks potentially. This problem is magnified if the number of leaf block in the index is huge.
Cheers
Riyaj Shamsudeen
Principal DBA,
Ora!nternals - http://www.orainternals.com<http://www.orainternals.com/> - Specialists in Performance, RAC and EBS
Blog: http://orainternals.wordpress.com/
Oracle ACE Director and OakTable member<http://www.oaktable.com/>
Co-author of the books: Expert Oracle Practices<http://tinyurl.com/book-expert-oracle-practices/>, Pro Oracle SQL, <http://tinyurl.com/ahpvms8> <http://tinyurl.com/ahpvms8> Expert RAC Practices 12c.<http://tinyurl.com/expert-rac-12c> Expert PL/SQL practices<http://tinyurl.com/book-expert-plsql-practices>
<http://tinyurl.com/book-expert-plsql-practices>
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