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Implicit Datatype Conversion + Histograms = Bad Execution Plan?
Earlier today I exchanged some tweets with @martinberx about some optimizer questions and after posting more information on the ORACLE-L list, I was able to reproduce what he was observing.The...
View ArticleMulti-Column Joins, Expressions and 11g
IntroductionI've already outlined in one of my previous posts that getting a reasonable cardinality estimate for multi-column joins can be tricky, in particular when dealing with correlated column...
View ArticleStar Transformation And Cardinality Estimates
If you want to make use of Oracle's cunning Star Transformation feature then you need to be aware of the fact that the star transformation logic - as the name implies - assumes that you are using a...
View ArticleTable Functions And Join Cardinality Estimates
If you consider the usage of Table Functions then you should be aware of some limitations to the optimizer calculations, in particular when considering a join between a Table Function and other row...
View ArticleDynamic Sampling On Multiple Partitions - Bugs
In a recent OTN thread I've been reminded of two facts about Dynamic Sampling that I already knew but had forgotten in the meantime:1. The table level dynamic sampling hint uses a different number of...
View ArticleColumn Groups - Edge Cases
Oracle 11g added Extended Statistics support for column groups in order to detect correlated columns for filter predicates using an equal comparison. Note that Oracle 11g also added the ability to use...
View ArticleForced Cursor Sharing And Virtual Columns
So you have that application that cannot be changed but makes use of some weird expressions that screw up the cardinality estimates of the optimizer. Consider this simple example: create table t as...
View ArticleHAVING Cardinality
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...
View ArticleTemp Table Transformation Cardinality Estimates - 1
Having published recently twonotes about the Temp Table Transformation highlighting the heuristics based decision and other weaknesses, for example regarding the projection of columns, it's time to...
View ArticleTemp Table Transformation Cardinality Estimates - 2
Continuing from the previous part - which was about the Temp Table Transformation and join cardinality estimates - using the same simple table setup here is a slight variation of the previously used...
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