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Supported join typesCockroachDB supports the following join types:
Inner joinsOnly the rows from the left and right operand that match the condition are returned.
Left outer joinsFor every left row where there is no match on the right,
Right outer joinsFor every right row where there is no match on the left,
Full outer joinsFor every row on one side of the join where there is no match on the other side,
Supported join conditionsCockroachDB supports the following conditions to match rows in a join:
Join algorithmsCockroachDB supports the following algorithms for performing a join:
Merge joinsTo perform a merge join of two tables, both tables must be indexed on the equality columns, and any indexes must have the same ordering. Merge joins offer better computational performance and more efficient memory usage than hash joins. When tables and indexes are ordered for a merge, CockroachDB chooses to use merge joins over hash joins, by default. When merge conditions are not met, CockroachDB resorts to the slower hash joins. Merge joins can be used only with distributed query processing. Merge joins are performed on the indexed columns of two tables as follows:
Hash joinsIf a merge join cannot be used, CockroachDB uses a hash join. Hash joins are computationally expensive and require additional memory. Hash joins are performed on two tables as follows:
Lookup joinsThe cost-based optimizer decides when it would be beneficial to use a lookup join. Lookup joins are used when there is a large imbalance in size between the two tables, as it only reads the smaller table and then looks up matches in the larger table. A lookup join requires that the right-hand (i.e., larger) table be indexed on the equality column. A partial index can only be used if it contains the subset of rows being looked up. Lookup joins are performed on two tables as follows:
You can override the use of lookup joins using join hints. Note: To make the optimizer prefer lookup joins to merge joins when performing foreign key checks, set the The output of
Inverted joinsInverted joins force the optimizer to use a join using a GIN index on the right side of the join. Inverted
joins can only be used with
See the cost-based optimizer examples for statements that use inverted joins. LATERAL joinsCockroachDB supports Apply joinsApply join is the operator that executes a lateral join if the optimizer is not able to de-correlate it (i.e., rewrite the query to use a regular join). Most of the time, the optimizer can de-correlate most queries. However, there are some cases where the optimizer cannot perform this rewrite, and Apply joins are inefficient because they must be executed one row at a time. The left side row must be used to construct the right side row, and only then can the execution engine determine if the two rows should be
output by the join. This corresponds to an Other types of joins supported by CockroachDB (e.g., hash join, merge join, and lookup join) are generally much more efficient. For example,
with a hash join, a hash table is constructed using rows from the smaller side of the join, and then the larger side of the join is used to probe into the hash table using the If you see an Performance best practices
See also
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