HASH JOIN
Short Description
A method for joining large data sets. The database uses the smaller of two data sets to build a hash table on the join key in memory. It then scans the larger data set, probing the hash table to find the joined rows.
Detailed Description​
As above - used on joining large data sets and can result in table scans occurring.
The hash table will be built in memory and if insufficient memory is allowed, it will spill to disk in the TEMPORARY tablespaces impacting performance.
Further Reading​
- Oracle Magazine - How to read an execution plan
- Oracle Tuning Guide - Explaining and Displaying Execution Plans
- Oracle Tuning Guide - Comparing Execution Plans Tutorial
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