In the previous post I've demonstrated an unexpected Nested Loop Join caused by an extreme data distribution. Although unexpected at first sight, the performance of the execution plan selected by the optimizer is decent - provided the estimates are in the right ballpark.Here is another case of an unexpected execution plan, this time about Merge Joins.
Recently I came across some interesting edge cases regarding the costing of joins. They all have in common that they result in (at first sight) unexpected execution plans, but only some of them are actual threats to performance.
Outer JoinsThe first one is about outer joins with an extreme data distribution. Consider the following data setup:
create table t1
rownum as id
, rpad('x', 100) as filler
, case when rownum > (more...)
Oracle 10g introduced the QB_NAME hint that can come handy in case hints need to be applied to more complex statements, in particular when possibly multiple layers of views / subqueries are involved.Jonathan Lewis has a older blog post that describes more details.Just in case you wonder why sometimes apparently the QB_NAME hint - along with all other hints that refer to the assigned query block name - seems to be ignored: One (more...)
Here is an interesting limitation to Exadata Smart Scans - if more than 254 columns from a table (not HCC compressed, more on that in moment) need to be projected, Smart Scans for that particular segment will be disabled and Exadata will fall back to conventional I/O. This means that the number of columns in the projection clause can make a significant difference to performance, since only Smart Scans allow taking advantage of offloading and (more...)
When performing aggregate GROUP BY operations an additional filter on the aggregates can be applied using the HAVING clause.Usually aggregates are one of the last steps executed before the final result set is returned to the client.However there are various reasons, why a GROUP BY operation might be somewhere in the middle of the execution plan operation, for example it might be part of a view that cannot be merged (or was hinted (more...)