Oracle SQL and-equal data access method Oracle Database Tips by Donald Burleson

When Oracle prepares an execution plan for SQL statements, he builds a costed decision tree with different access methods.  The and-equal is one such access method which was widely used in the rule-based optimizer (RBO) but is now largely obsolete. (the and-equal hint is still supported in the CBO).

Using ?and_equal? Hints To Concatenate Indexes:

 

Index1           column1_primary key

Index2           column2

Index3           column1        column2 ?

Index4           column2        column1 ?

In this case index3 and 4 were created for multiple column queries with some lead columns on 1 and some lead columns on 2.  In this case Index 3 and 4 could be removed and multi-column queries that use column1 and 2 can use the ?and_equal? hint to cause the optimizer to use index1 and index2 before accessing the table with rowids.  Using the ?and_equal? hint does have additional overhead since two indexes must be read.  You have to balance the overhead of two reading two indexes against the overhead of the additional indexes. 

I also find cases where Index3 and Index4 were added to support queries that contained both column1 and column2 in separate WHERE clauses.  These indexes were added because in some queries column1 is more restrictive (so it is the lead column) and in other queries column2 is more restrictive (and thus the lead column).  In fact either Index3 or Index4 (or using the ?and_equal? hint) will satisfy the query and will have equivalent performance.  You are also relying on the optimizer to pick the correct index to use, which is unlikely.  Having the lead column the most restrictive column will not affect the index performance.  In this case you can drop either Index3 or Index4 and not affect query performance. 

Remember that leading columns are only important in that they must be used in the WHERE clause for the index to be used.  Oracle 9i introduced the index skip scan that allows you to use an index where  the lead column is not in the where clause, but this requires more overhead than performance benefit.

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This page has excellent notes on the and-equal access method:

SELECT *
FROM   my_table
WHERE  column5 = 230
AND    column8 = 'BLORT'

SELECT STATEMENT
TABLE ACCESS MY_TABLE BY ROWID
AND-EQUAL
NON-UNIQUE INDEX RANGE SCAN MY_TABLE_I5
NON-UNIQUE INDEX RANGE SCAN MY_TABLE_I8

The and-equal access method scans all nominated single column indexes used in AND col1 = xxx and col2=yyy, where both col1 and col2 have non-unique single key indexes on them.

The and-equal hint is specified in this form:

/*+ AND_EQUAL ( table index index [index] [index] [index] ) */

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