Hash Join
Short Description
The Hash Join node builds a hash table from the right table, then other table is then scanned and compared to the hash table checking for matches
Detailed Description​
The Hash Join node builds a hash table from the right table or relation, then scans the other table checking each value against the hash table checking for matches. If the size of the hash table is bigger than work_mem, then the job is split up and each result is written to disk. Writing out to disc has a large negative impact on execution time.
Additional Links​
- PostgreSQL Documentation - Using Explain
- pgAnalyze - Explain
- eTutorials - Understanding How PostgreSQL Executes a Query
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