Sort Merge Join Oracle Tips by Burleson Consulting

A sort merge join is an operation that retrieves two set of rows from the target tables and then sorts each row set by the join column. The merge phase combines each row from one set with the matching rows from the other, returning the intersection of the row sets (see Figure 3-9).

Figure 9: A SQL sort merge join

There are several permutations of the sort merge join, an outer merge join, an anti-merge join, and a semi-merge join. These permutations alter the merge phase from the goal of finding the intersections of the row sets to finding only those rows that meet the conditions of the SQL where clause.

STAR Query Join

With this new join method, data warehouse queries can run at blistering speeds, in some cases dozens of times faster than the original query. The STAR schema design involves creating a main fact table that contains all of the primary keys in the related data warehouse tables. This massive denormalization of the database structure means that just about any query against the STAR schema is going to involve the joining of many large tables, including a large fact table and many smaller reference tables. To invoke the STAR query path, the following characteristics must be present:

• There must be at least three tables being joined, with one large fact table and several smaller dimension tables.

• There must be a concatenated index on the fact table with at least three columns, one for each of the table join keys.

• You must verify with an explain plan that the NESTED LOOPS operation is being used to perform the join.

Essentially, Oracle follows a simple procedure for processing STAR queries. Oracle will first service the queries against the smaller dimension tables, combining the result set into a Cartesian product table that is held in Oracle memory. This virtual table will contain all of the columns from all of the participating dimension tables. The primary key for this virtual table will be a composite of all of the keys for the dimension tables. If this key matches the composite index on the fact table, then the query will be able to process very quickly.

Once the sum of the reference tables has been addressed, Oracle will perform a nested loop join of the intermediate table against the fact table. This approach is far faster than the traditional method of joining the smallest reference table against the fact table and then joining each of the other reference tables against the intermediate table. The speed is a result of reducing the physical I/O. The indexes are read to gather the virtual table in memory, and the fact table will not be accessed until the virtual index has everything it requires to go directly to the requested rows via the composite index on the fact table.

Connect by Join

A connect by join is an operation for the retrieval of rows in a hierarchical order for a SQL query that contains a connect by clause. The connect by clause is seldom used in most SQL statements. To see an example of the connect by clause, please see the plan.sql code that we use in this book to display SQL execution plans.

A Comparisons of Join Speeds

The reason that Oracle offers several methods for joining tables is their experience that the nature of the tables greatly impacts the efficiency of the joining. By far, the three most popular join methods are the nested loop join, the sort merge join, and the hash join, in that order.

The sort merge join and the hash join are very similar in terms of function, while the nested loop join is quite different from the other join techniques. Table 3-1 summarizes the relative advantages of each join technique.

Join Type	Advantages	Sorting	Large Row Subsets	Index Required?
Sort merge join Hash join	Faster throughput	Yes	Fast	No
Nested loop join	Faster response time	No	Slow	Yes

Table 1: Relative Advantages of Join Types

In sum, the nested loop join provides a faster response time for SQL queries that return small row subsets from each table. For large row subsets, the sort merge and hash join may result in faster execution time. Again, we will return to these join methods in detail in Chapter 16.

Now that you have a basic understanding of the Oracle join methods, let’s explore the methods used by Oracle to sort a result set.

This is an excerpt from "Oracle High-Performance SQL Tuning" by Donald K. Burleson, published by Oracle Press.