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hash join (left outer)

Short Description

The hash join (left outer) builds a hash table in memory based on the left table, then scans the right table looking for matches.

Detailed Description

The hash join (left outer) builds a hash table in memory based on the left table and then scans the right table looking for matches.

Being a LEFT join, all rows from the left table are returned along with matching rows from the right table. Unmatched rows from the right table are not returned.

Hash joins are computationally expensive and require additional memory for the hash table to be built in, which is why a merge join is preferred if possible.

If the hash table is too big for memory, it will spill to disk which will impact performance.

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