Join orderperformance implications
ofJoin orderrules
of thumbThe optimizer usually makes a good
choice about join order. This section discusses the performance implications
of join order.
In a join operation involving two tables, scans the tables
in a particular order. accesses rows in one table first, and this
table is now called the outer table.
Then, for each qualifying row in the outer table, looks for
matching rows in the second table, which is called the inner table.
accesses the outer table once, and the inner table probably
many times (depending on how many rows in the outer table qualify).
This leads to a few general rules of thumb about join order: