Indexes

Indexes - What are they and how can I use them?

An index helps to speed up selection from the database. An index is a sorted copy of selected database table fields.

The primary index is always automatically created in an ABAP-based SAP system. It consists of the primary key fields of the database table. This means, for each combination of the index fields exists a maximum of one record in the table. This kind of index is called a UNIQUE index.

If the primary index cannot be used to determine selection result, (for example, the WHERE condition does not contain any primary index fields), the system searches the whole table. To prevent this, and determine the selection result by searching through a restricted number of database records, you can create a secondary index.

However, you should not define an index for all possible fields in the WHERE condition.

Creating a secondary index

You can use the transaction ABAP Dictionary Change → Indexes... → Create to create an index. To make the index unique, select UNIQUE. To specify the fields that will comprise the index, choose "Choose fields". Then save and activate the index.

When to create an index


It is worth creating a secondary index when:

1. You want to select table entries based on fields that are not contained in an index, and the response times are very slow.
   The EXPLAIN function in the SQL trace shows which index the system is using. You can generate a list of the database queries involved in an action by entering Transaction ST05 and choosing Trace on → Execute action → Trace off → List trace. If you execute the EXPLAIN SQL function on a EXEC, REEXEC, OPEN, REOPEN or PREPARE statement, the system returns a list containing the index used in the database query.
2. The field or fields of the new secondary index are so selective that each index entry corresponds to at most 5% of the total number of table entries. Otherwise, it is not worth creating the index.
3. The database table is accessed mainly for reading entries.
   

Using an index consisting of several fields

Even if an index consists of several fields, you can still use it when only a few of the fields actually appear in the WHERE clause. The sequence in which the fields are specified in the index is important. You can only use a field in the index if all of the preceding fields in the index definition are included in the WHERE condition.

An index can only support search criteria which describe the search value positively, such as EQ or LIKE. The response time of conditions including NEQ is not improved by an index.

Optimal number of fields for an index

An index should only consist of a few fields; as a rule, no more than four. This is because the index has to be updated each time you change its fields in a database operation.

Fields to include in an index

1. Include fields that are often selected and have a high selectivity. In other words, you need to check the proportion of the table entries that can be selected with this field. The smaller the proportion, the more selective the field. You should place the most selective fields at the beginning of the index.
2. If all of the fields in a SELECT statement are contained in the index, the system does not access the data a second time following the index access. If there are only a few fields in the SELECT statmeent, you can improve performance significantly by including all of these fields in the index.
3. You should not include a field in an index if its value is initial for most of the table entries.
   

Optimal number of indexes for a table

You should not create more than five indexes for any one table because:

1. Whenever you change table fields that occur in the index, the index itself is also updated.
2. The amount of data increases.
3. The optimizer has too many chances to make mistakes by using the 'wrong' index.
   
   

If you are using more than one index for a database table, ensure that they do not overlap.

Avoiding OR conditions

The optimizer generally stops if the WHERE condition contains an OR expression. In other words, it does not evaluate the fields in the OR expression with reference to the index.
An exception to this are OR statements standing on their own. Try to reformulate conditions containing an OR expression for one of the indexed fields. For example, replace:

SELECT * FROM SPFLI
             WHERE CARRID = 'LH'
             AND  (CITYFROM = 'FRANKFURT' OR  CITYFROM = 'NEW YORK').

with:

SELECT * FROM SPFLI
             WHERE (CARRID = 'LH' AND CITYFROM = 'FRANKFURT')
             OR    (CARRID = 'LH' AND CITYFROM = 'NEW YORK').

Problems with IS NULL

The value NULL is not stored in the index structure of some database systems. The consequence of this is that the index is not used for that field.