When tables and indexes are ordered for a merge, CockroachDB chooses to use merge joins over hash joins, by default.

Merge joins offer better computational performance and more efficient memory usage than hash joins. To perform a merge join of two tables, both tables must be indexed on the equality columns, and any indexes must have the same ordering. SQL Anywhere 11.0.Note: NATURAL is supported for compatibility with PostgreSQL Cockroach Labs discourages use in new applications because its results can silently change in unpredictable ways when new columns are added to one of the join operands.ĬockroachDB supports the following algorithms for performing a join: Send feedback about this page using email.Ĭopyright © 2009, iAnywhere Solutions, Inc. 'Extra large', Color, 50, UnitPrice, Photo You can demonstrate the difference in the two queriesīy inserting some new products into the Products table for which there are no orders and then executing the queries again. In this way, the test of Quantity is part of the outer join condition. In this example, the query is almost certainly written incorrectly it should instead be: SELECT * This rewritten form is the query that the database server optimizes. The query is semantically equivalent to:įROM Products p KEY JOIN SalesOrderItems s The problem with this query is that the predicate in the WHERE clause eliminates any product with no orders from the resultīecause the predicate s.Quantity > 15 is interpreted as FALSE if s.Quantity is NULL. The following query lists products and their corresponding orders for larger quantities the LEFT OUTER JOIN ensures thatĪll products are listed, even if they have no orders:įROM Products p KEY LEFT OUTER JOIN SalesOrderItems s SELECT * FROM SalesOrderItems s JOIN Products p ON (p.ID = s.ProductID) The following SELECT statements show how the query is rewritten after a rewrite optimization: SELECT * FROM SalesOrderItems s LEFT OUTER JOIN Products p ON (p.ID = s.ProductID) The ProductID column is declared not NULL and the SalesOrderItems table has the following foreign key: 'FK_ProductID_ID' ('ProductID') REFERENCING 'Products' ('ID'). One or more table expressions would be eliminated, making this optimization applicable.įor the query below, for each row of the SalesOrderItems table there is exactly one row that matches the Products table because The query WHERE clause may include conditions that restrict the output such that all null-supplying rows from

This rewrite optimization can apply to an outer join query when the query refers to one or more views that are written using Is true, there are no null-supplied rows and the OUTER JOIN is equivalent to an INNER join.

the null-supplying side of an OUTER JOIN returns exactly one row for each row from the preserved side.

Making the query semantically equivalent to an inner join. This predicate is null-intolerant, any all-NULL row that would be produced by the OUTER join is eliminated from the result,

a null-intolerant predicate referencing columns of the null-supplying tables is present in the query WHERE clause.

Greater degrees of freedom when performing join enumeration.Ī LEFT or RIGHT OUTER JOIN is converted to an INNER JOIN if one of the following conditions is true: To convert LEFT or RIGHT outer joins to INNER JOINs whenever possible, since INNER JOINs are commutable and give the optimizer So, the optimizer looks for opportunities Require that preserved tables must precede null-supplying tables in any join strategy. The query execution engine's algorithms for computing LEFT or RIGHT OUTER JOINs Of a right-deep nested outer join expression. The only exception to this rule is the existence The optimizer generates a left-deep processing tree for its access plans.

Postgresql left inner join