DML restart and tracked columns

Which columns are rely tracked by oracle to grantee write consistency and if needed trigger a DML restart when using the default statement-level consistency ?

Sometimes you may see strange ,unexpected behavior and write consistency bugs because not all the columns that are supposed to be tracked ,are really tracked !

Just check the following articles :

A bug from 2011 (still there in 18c) :

A Bug from 2006  (still there in 12.2) :

So how to know it the columns that are supposed to be tracked really are ? I’m going only to treat UPDATE and MERGE statement here.

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Oracle trace events hunting : Events annotations/Events Sniffing

Based on my previous blog posts :

I created two files that can help us to quickly extract Events checked in specific oracle core function :

Using those files we can for example :

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Oracle trace events hunting : Undocumented events/Filling the gaps

In my last blog post Oracle trace events hunting : dbgdChkEventIntV i talked about how we can extract events that are checked in specific oracle core function by analyzing the arguments passed to dbgdChkEventIntV function. I used for that a mapping file called dbgdChkEventIntV_event_list.txt   (Basically mapping EventId to actual Event Name)

When we analyze the mapping file we observe something :

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The different events seems to have a sequential Event ID  (stored in an array ?)  ! So what is stored in slot 1160002,116004 ,116007 etc ? Let’s check !

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Oracle trace events hunting : dbgdChkEventIntV

Few days ago i published a blog post talking about write consistency & DML restart  . During my investigation i was interested in the function “dmltrace” and i noticed that this function was instrumented with the dbk*/dbg*  debug functions introduced in 11g.

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The question is how to enable this tracing facility ?  Which events are checked by that function ?

We know how to extract the trace events number from the old ksdpec function (kernel service debug internal errors parser post event and check trigger condition using of course 😀  ) thank to Dennis Yurichev see here and here. 

We also know how to extract it from dbkdChkEventRdbmsErr (DB kernel debug check event of RDBMS error) thanks to Yong Huang see here.

But here the function is instrumented differently !

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Write consistency and DML restart

Few weeks ago Tanel Poder published a great video talking about Oracle’s write consistency , DML restarts and demonstrating also how we can detect them.He also published a script that allow finding UPDATE/DELETE statements that hit the write consistency issue & have to restart under the hood. But as he stated it uses V$SQL_PLAN_MONITOR, so requires Diag+Tuning pack licenses.

The purpose of this blog post is to show another way to detect statements hitting the write consistency issue.

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