NestedLoopsJoin computes the join of its left and right-hand sides by completely reading the right-hand side for each row of the left-hand side. (The syntactic order of tables in the query does not matter because the optimizer chooses the appropriate join order for each block in the request.)
The optimizer may choose NestedLoopsJoin if the join condition does not contain an equality condition, or if the statement is being optimized with a first row optimization goal (that is, either the optimization_goal option is set to First-Row, or FASTFIRSTROW is specified as a table hint in the FROM clause.
Since NestedLoopsJoin reads the right-hand side many times, it is very sensitive to the cost of the right-hand side. If the right-hand side is an index scan or a small table, then the right-hand side can likely be computed using cached pages from previous iterations. However, if the right-hand side is a sequential table scan or an index scan that matches many rows, then the right-hand side needs to be read from disk many times. Typically, NestedLoopsJoin is less efficient than other join methods. However, NestedLoopsJoin can provide the first matching row quickly compared to join methods that must compute their entire result before returning.
NestedLoopsJoin is the only join algorithm that can provide sensitive semantics for queries containing joins. This means that sensitive cursors on joins can only be executed with NestedLoopsJoin.
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