Indexes 
Indexes
You can search a table for data records that satisfy certain search criteria faster using an index.
An index can be considered a copy of a database table that has been reduced to certain fields. This copy is always in sorted form. Sorting provides faster access to the data records of the table, for example using a binary search. The index also contains a pointer to the corresponding record of the actual table so that the fields not contained in the index can also be read.
The primary index is distinguished from the secondary indexes of a table. The primary index contains the key fields of the table and a pointer to the non-key fields of the table. The primary index is created automatically when the table is created in the database.
Table SCOUNTER in the
flight model contains the assignment of the carrier counters to airports. The primary index on this table therefore consists of the key fields of the table and a pointer to the original data records.
You can also create further indexes on a table in the ABAP Dictionary. These are called secondary indexes. This is necessary if the table is frequently accessed in a way that does not take advantage of the sorting of the primary index for the access. Different indexes on the same table are distinguished with a three-place
index identifier.
All the counters of carriers at a certain airport are often searched for flight bookings. The airport ID is used to search for counters for such an access. Sorting the primary index is of no use in speeding up this access. Since table SCOUNTER has a large number of entries, a secondary index on the field AIRPORT (ID of the airport) must be created to support access using the airport ID.
The optimizer of the database system decides whether an index should be used for a concrete table access (see
How to Check if an Index is Used?). This means that an index might only result in a gain in performance for certain database systems. You can therefore define the database systems on which an index should be created when you define the index in the ABAP Dictionary (see Creating Secondary Indexes.)All the indexes existing in the ABAP Dictionary for a table are normally created in the database when the table is created if this was not excluded in the index definition for this database system.
If the index fields have key function, that is if they already uniquely identify each record of the table, an index can be defined as a
unique index.See also:
What to Keep in Mind when Creating Secondary Indexes
