Cartesian Join with Hierarchical Query
How to avoid wrong cardinalities in cartesian joins Continue Reading
cost based optimizer
JOIN CARDINALITY MISESTIMATE in Oracle 12.2
Wrong join cardinality estimate due to adaptive dynamic sampling Continue Reading
Cardinality Estimation for “MINUS” Operator
Mind inaccurate cardinality estimations when using MINUS operator in subqueries. Continue Reading
Cost-Based OR Expansion Transformation
OR expansion not working properly in Oracle 12.2 Continue Reading
Large TCHK Allocations in Shared Pool
Large TCHK allocations in shared pool caused by create table as select Continue Reading
Bind Variable Peeking vs. Parameter Sniffing
Comparison of bind variable peeking in Oracle with parameter sniffing in SQL Server Continue Reading
Comparison of a Cardinality Estimate in Oracle and SQL Server
This blog post is a short observation on assumptions that Oracle and SQL server optimizers make for estimating cardinality of a rowset obtained after applying an equality predicate which contains different columns of the same table. Continue Reading
Large Memory Allocations due to Query Parsing
Troubleshooting high PGA memory allocations which escalated in ORA-04036 error. High memory consuption was related to a parsing problem. Continue Reading
Tables Containing “Current” and “Historical” Rows
If your data model forsees storing “current” and “historical” rows in the same table, there is a chance that there will be queries doing self-joins drived by the “current” rows. This model can give rise to wrong join cardinality estimates applied on skewed data distributions. The more emphasized the skew is, the larger is the error. Table partitioning turns out to be a good solution in such cases. Continue Reading
Density Calculation with Hybrid Histograms
This blog post describes how the calculation of new density has modified with the introduction of hybrid histograms in Oracle 12c. Continue Reading