Non-intrusive SQL Trace instrumentation on legacy PL/SQL code
Legacy PL/SQL code with intermittent performance degradation.
To improve the performance of this code, the first step is to diagnose it. But this code has no instrumentation whatsoever, it is in Production, and rolling any code into Production usually requires rigorous testing. So, whatever method we implement has to be light-weight and as safe as possible.
Using Oradebug is not a viable solution mainly for two reasons: It misses the “head” of the transaction, so we may not get to know the SQL taking longer; and second, internal procedures from finding about the issue, reporting it, then acting on it may take from several minutes to hours.
- Identify which PL/SQL libraries are known to be problematic in terms of intermittent performance degradation.
- At the beginning of each callable PL/SQL Procedure or Function, add a call to DBMS_APPLICATION_INFO.SET_MODULE to set some appropriate MODULE and ACTION, for example “R252, LOAD”. Call this API also at the end, to NULL out these two parameters. This code change is very small and safe. It introduces practically no overhead. It simply labels every SQL executed by the PL/SQL library with some MODULE and ACTION that uniquely identify the code of concern.
- Activate SQL Trace on the module/action that needs to be traced, by calling DBMS_MONITOR.SERV_MOD_ACT_TRACE_ENABLE, passing parameters SERVICE, MODULE and ACTION. With this API request a SQL Trace to be generated with WAITs and BINDs (binds are optional but desirable). Once these traces are no longer needed (reviewed by someone), turn SQL Trace off using API DBMS_MONITOR.SERV_MOD_ACT_TRACE_DISABLE.
- Once the SQL Trace is produced, generate a TKPROF on it. You may want to include parameter “sort=exeela fchela”. This way you get the slower SQL at the top of the TKPROF report.
- With SQL Trace and TKPROF, identify the slower SQL and use SQL Monitor and/or SQL XTRACT to get more granular diagnostics (you need to identify SQL_ID). On any given PL/SQL library, it is common that 1~5 SQL statements consume > 80% of the Elapsed Time. Focus on these large consumers.
Producing a SQL Trace with EVENT 10046 level 8 or 12 is very useful to properly diagnose the code on a PL/SQL library which performs poorly. A follow-up on the slower SQL with SQL Monitor and/or SQLT XTRACT is in order. The method presented above is very easy and safe to implement.