Insane DBA...

I’ve moved my blog from https://insanedba.blogspot.com to https://dincosman.com Please update your bookmarks and follow/subscribe at the new address for all the latest updates and content. More up-to-date content of this post may be available there. INTRO   In one of our critical OLTP database, we faced " Latch: cache buffer chains " wait event issue(a long running query) on a commonly used page. Long running query was called inside a PL/SQL package function. To alleviate the situation, first we changed the function to a deterministic function and rewrite the query's function call into a scalar subquery allows Oracle to use scalar subquery caching to optimize the number of function calls. As these cautions could not cure the issue and segment access statistics were not even, we changed table partitioning strategy from list partitioned to hash-partitioned. For the problematic query issue was solved, but a batch job which completes in 5 minutes before, started to ...

I’ve moved my blog from https://insanedba.blogspot.com to https://dincosman.com Please update your bookmarks and follow/subscribe at the new address for all the latest updates and content. More up-to-date content of this post may be available there. Previously on my blog post ( Sometimes a histogram existence may mislead to a suboptimal plan ), I analyzed a poorly performing query in detail and detected that sometimes a histogram existence may mislead to a suboptimal plan. I have listed some possible solutions.  But what i applied as a solution was to emulate a partial/filtered index in Oracle.   I know partial indexes concept from Postgresql database, it is also available on MS SQL and some others.  What is Partial Index? ... A partial index is an index built over a subset of a table; the subset is defined by a conditional expression (called the predicate of the partial index). The index contains entries only for those table rows that satisfy the predicat...

Manipulating table statistics for the statement period I’ve moved my blog from https://insanedba.blogspot.com to https://dincosman.com Please update your bookmarks and follow/subscribe at the new address for all the latest updates and content. More up-to-date content of this post may be available there. On my blog post " Who makes the decisions? I thought it was Optimizer ", I used parallel hint to force a direct path read. This time I will try to make use of table_stats hint to force a direct path read. There is not much documentation about the usage of this hint. Roger Macnicol's blog post ( Correct syntax for the table_stats hint  ) offers up some useful information about it. These are the variables that play a role in direct path read decision.  STT stands for small table threshold = _db_block_buffers * 0.02 =  3875820 * 0.02 = 77516 MTT stands for medium table threshold = _small_table_threshold * 5 = 77516 * 5 = 387580 VLOT stands for Very Large Object Thres...

The reason behind is NLS_SETTINGS I’ve moved my blog from https://insanedba.blogspot.com to https://dincosman.com Please update your bookmarks and follow/subscribe at the new address for all the latest updates and content. More up-to-date content of this post may be available there. I generally use Stew Ashton's below query to summarize reasons why cursors are not shared.  In one of our production database, the amount of child cursor count related to "LANGUAGE_MISMATCH" attracted my attention and I tried to make clear the reason behind. According to the  Database Hang With 'cursor: mutex X' Contention Due To High Version Count Under LANGUAGE_MISMATCH (Doc ID 2542447.1) , whenever there are different NLS settings that get changed in different combinations in every execution, it creates new child cursors and the version count increases with LANGUAGE_MISMATCH. Exactly the same queries and PL/SQL packages are called from different applications . Differ...