Database query tuning is mostly about getting better plans. Mostly, but not always. Sometimes, the problem has nothing to do with the plan, and you might need to get a bit creative to find a solution. In this recent case a query was showing a decent performance when running from SQL Developer, but it took about 5 times longer to complete when running from R. The plan was the same, so I knew that it was irrelevant. The R session wasn’t showing as active most of the time, so it was fairly clear that the problem was fetching data — i.e. it was fetching too few rows at a time which lead to a large number of roundtrips, and consequently, high waits on “idle” event “SQL*Net message from client”.
On one of the databases I’m looking after (126.96.36.199, Solaris, non-RAC), several different INSERT statements (all into tablespaces with manually managed segments) suffer from occasional hiccups. The symptoms are always the same: in one of the sessions, the INSERT gets stuck doing lots of single-block I/O against one of the indexes on the inserted table, and if other sessions are running similar INSERTs, they hang on enq: TX – index contention. The situation can last just a few seconds, but sometimes it’s much longer than this (several minutes), in which case the impact on the application is quite serious.
In the database world (especially among the database developers) a commit is often viewed as some sort of a virtual “save” button — i.e. it’s something that you need to do to make your changes permanent. This is one of the reasons why developers are often happy to commit work as soon as they get a chance (even if it’s not properly finished), so that they wouldn’t lose their changes in case of an error. Unfortunately, they don’t always think the whole thing through — restarting a process interrupted half-way may be much more difficult than re-doing everything from scratch.
SQL performance can degrade for many reasons, some of most common are:
– plan changes
– data skewness
– low caching efficiency
– data growth
All these factors are relatively well known. A somewhat less common, although not exceptionally rare scenario, is read consistency overhead due to concurrent DML against queried tables. Because of being less common, this scenario is often overlooked, which leads to false diagnoses (and eventually to “fixes” that can do more harm than good).