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Oracle Join Methods

Posted by Vimal Mohan on May 14, 2011

The method used to join tables with each other in Oracle.

Hash Joins

Hash joins are used for joining large data sets. The optimizer uses the smaller of the two tables or data sources to build a hash table, based on the join key, in memory. It then scans the larger table, and performs the same hashing algorithm on the join column(s). It then probes the previously built hash table for each value and if they match, it returns a row.

Nested Loops joins

Nested loops joins are useful when small subsets of data are being joined and if there is an efficient way of accessing the second table (for example an index look up). For every row in the first table (the outer table), Oracle accesses all the rows in the second table (the inner table). Consider it like two embedded FOR loops. In Oracle Database 11g the internal implementation for nested loop joins changed to reduce overall latency for physical I/O so it is possible you will see two NESTED LOOPS joins in the operations column of the plan, where you previously only saw one on earlier versions of Oracle.

Sort Merge joins

Sort merge joins are useful when the join condition between two tables is an inequality condition such as, <, <=, >, or >=. Sort merge joins can perform better than nested loop joins for large data sets.

The join consists of two steps: 1) Sort join operation: Both the inputs are sorted on the join key. 2) Merge join operation: The sorted lists are merged together.

A sort merge join is more likely to be chosen if there is an index on one of the tables that will eliminate one of the sorts.

Cartesian join

The optimizer joins every row from one data source with every row from the other data source, creating a Cartesian product of the two sets. Typically this is only chosen if the tables involved are small or if one or more of the tables does not have a join conditions to any other table in the statement. Cartesian joins are not common, so it can be a sign of problem with the cardinality estimates, if it is selected for any other reason. Strictly speaking, a Cartesian product is not a join.

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