Original Post can be viewed at Optimizer Choosing Nested-Loop Joins Instead of Hash-Joins

In one of my databases, one application query suddenly started to pick Nested-Loop joins instead of Hash-Joins and took almost 6 hours to complete which gets completed in less than 10 secs with Hash-Joins. The same query in another similar database with same configuration and same data is doing fine and using hash joins. There is [...]

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Original Post can be viewed at Plan Stability using Sql Profiles and SQL Plan Management

PLAN STABILITY How many times you have noticed a query using Index X when you wanted it to use index Y or query performing Nested Loop join when Hash Join would have completed the query much faster.Or take a scenario when the application suddenly starts using wrong plan after database restart. To solve all these [...]

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Original Post can be viewed at V$SQL_SHARED_CURSOR in 11.2.0.2

Oracle 11.2.0.2 has introduced new column “REASON” to V$SQL_SHARED_CURSORS. I came across this extremely useful column while debugging multiple child cursor issue (11.2.0.2 seems to have too many bugs which can cause multiple child cursors). As per documentation , REASON column has been introduced in 11.2.0.2 REASON - CLOB Child number, id, and reason the [...]

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Original Post can be viewed at 11gR2: Monitoring Real Time SQL Plan Execution from Oracle Enterprise Manager

Ever since 11gR2 has been released, it has created a lot of buzz in Oracle Blogosphere with various posts on 11g Release 2 (11gR2 ) New Features primarily ASM New features. I would like to introduce you all to a feature which has been incorporated in 11gR2 Oracle Enterprise Manager to display Real-Time SQL Monitoring information . [...]

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Original Post can be viewed at dbms_stats.copy_table_stats does not alter low/high value

I was working on a performance issue arising due to missing table stats on a new partition for large Partitioned table.This was a 10.2.0.4 database on IBM AIX 5L. So as to resolve the issue , I used dbms_stats.copy_table_stats to copy the stats to new parition from the previous partition. EXEC DBMS_STATS.COPY_TABLE_STATS (‘owner’, ‘tabname’, ‘SourcePart’, [...]

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Original Post can be viewed at Index Access best approach?? Not Always…

There is a common misconception that Index access is the best access method for query execution. This approach leads to people concentrate on removing Full Table Scans (FTS) from the Query execution plan. I recently worked on a performance tuning issue where I found such scenario. Basically customer was trying to generate report for 3 [...]

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Original Post can be viewed at 11g New Feature: Database Replay

Database Replay: In critical production systems, before making changes to the environment( Hardware or software related), lots of testing is required in test systems to know the effect of those changes and this usually takes lots of time to test for the stability and effects of the change. In Oracle 11g this thing is simplified [...]

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Original Post can be viewed at ORA- 4031 – A Case Study

Today I will be taking up one ORA-4031 issue we faced on one of our client database. Please note that SQL statements mentioned below have been changed and does not reveal any confidential information. Our client was facing ORA-4031 on a 9.2 database.This was a new Database which had gone Live recently. Following error messages [...]

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Original Post can be viewed at Tracing Sessions with Event 10046

10046 Event is used to trace SQL statements (similar to sql_trace=true) with additional details depending on the tracing level which is specified. 10046 EVENT levels: 1 – Enable standard SQL_TRACE functionality (Default) 4 – As Level 1 PLUS trace bind values 8 – As Level 1 PLUS trace waits 12 – As Level 1 PLUS [...]

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Original Post can be viewed at Application Design and ORA – 4031

Much has been written on designing good application (which uses bind variables) so as to have better performance and avoid shared pool memory issues. Tom Kyte explains this on one of his post consequences of not using bind variables If you do not use bind variables and you flood the server with hundreds/thousands of unique [...]

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Original Post can be viewed at Tweaking _Shared_pool_reserved_min_alloc and ORA – 4031

Here comes one more article dedicated towards diagnosing ORA- 4031 and related parameters. While googling on ORA-4031 error or working on TAR with Oracle Support you will most likely hit this parameter “_Shared_pool_reserved_min_alloc “ and find a advice for altering this parameter to 4000 or so. And many times you go and implement it as [...]

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Original Post can be viewed at HW enqueue contention with LOB

DefinitionEnqueues are local locks that serialize access to various resources. Enqueue wait event indicates a wait for a lock that is held by another session (or sessions) in an incompatible mode to the requested mode. HW Enqueue – High Watermark enqueue is acquired when a segment’s high water mark is moved, which will normally happen [...]

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Original Post can be viewed at Shared Sub Pools

Today I will discuss about Shared pool subpools which have been introduced in release 9iR2 and higher (To be precise >9.2.0.5) and can be controlled by a hidden parameter “_kghdsidx_count “ Most of you would not have come across this term/parameter unless you have faced ORA-4031 error. As you all are aware that Shared Pool [...]

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Original Post can be viewed at Diagnostics For Database Hang

Many times Oracle DBA’s are in a situation when the database is hung and does not seem to be responding. In some scenarios, state is such that you cannot even connect to the sqlplus session. Majority people restart the database (Sometimes I wonder if this is due to the fact that most of us started [...]

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