Purpose of Unique key index while declaring the table with data statement

[Former Member]

HEllo Abapers,

GO thru the below code

DATA : BEGIN OF LINE,

INDEX TYPE I,

TEXT(8) TYPE C,

END OF LINE.

DATA ITAB LIKE SORTED TABLE OF LINE WITH UNIQUE KEY INDEX.

Here what is the purpose of defining sorted table of ine with unique key index.

Pelase clariy me,,,

[Former Member]

Unique Indexes

An entry in an index can refer to several records that have the same values for the index fields. A unique index does not permit these multiple entries. The index fields of a unique index thus have key function, that is they already uniquely identify each record of the table.

The primary index of a table is always a unique index since the index fields form the key of the table, uniquely identifying each data record.

You can define a secondary index as a unique index when you create it. This ensures that there are no double records in the table fields contained in the index. An attempt to maintain an entry violating this condition in the table results in termination due to a database error.

The accessing speed does not depend on whether or not an index is defined as a unique index. A unique index is simply a means of defining that certain field combinations of data records in a table are unique.

A unique index for a client-dependent table must contain the client field.

Awrd Points If Useful

[Former Member]

this document will help you to understand the table types

Internal table types

Like all local data types in programs , you define internal tables using the TYPES statement. If you do not refer to an existing table type using the TYPE or LIKE addition, you can use the TYPES statement to construct a new local internal table in your program.

TYPES <t> TYPE|LIKE <tabkind> OF <linetype> [WITH <key>] [INITIAL SIZE <n>].

After TYPE or LIKE, there is no reference to an existing data type. Instead, the type constructor occurs:

<tabkind> OF <linetype> [WITH <key>]

The type constructor defines the table type <tabkind>, the line type <linetype>, and the key <key> of the internal table <t>.

You can, if you wish, allocate an initial amount of memory to the internal table using the INITIAL SIZE addition.

Table type

You can specify the table type <tabkind> as follows:

Generic table types

INDEX TABLE

For creating a generic table type with index access.

ANY TABLE

For creating a fully-generic table type.

Data types defined using generic types can currently only be used for field symbols and for interface parameters in procedures . The generic type INDEX TABLE includes standard tables and sorted tables. These are the two table types for which index access is allowed. You cannot pass hashed tables to field symbols or interface parameters defined in this way. The generic type ANY TABLE can represent any table. You can pass tables of all three types to field symbols and interface parameters defined in this way. However, these field symbols and parameters will then only allow operations that are possible for all tables, that is, index operations are not allowed.

Fully-Specified Table Types

STANDARD TABLE or TABLE

For creating standard tables.

SORTED TABLE

For creating sorted tables.

HASHED TABLE

For creating hashed tables.

Fully-specified table types determine how the system will access the entries in the table in key operations. It uses a linear search for standard tables, a binary search for sorted tables, and a search using a hash algorithm for hashed tables.

Line type

For the line type <linetype>, you can specify:

Any data type if you are using the TYPE addition. This can be a predefined ABAP type, a local type in the program, or a data type from the ABAP Dictionary. If you specify any of the generic elementary types C, N, P, or X, any attributes that you fail to specify (field length, number of decimal places) are automatically filled with the default values. You cannot specify any other generic types.

Any data object recognized within the program at that point if you are using the LIKE addition. The line type adopts the fully-specified data type of the data object to which you refer. Except for within classes, you can still use the LIKE addition to refer to database tables and structures in the ABAP Dictionary (for compatibility reasons).

All of the lines in the internal table have the fully-specified technical attributes of the specified data type.

Key

You can specify the key <key> of an internal table as follows:

[UNIQUE|NON-UNIQUE] KEY <col1> ... <col n>

In tables with a structured line type, all of the components <coli> belong to the key as long as they are not internal tables or references, and do not contain internal tables or references. Key fields can be nested structures. The substructures are expanded component by component when you access the table using the key. The system follows the sequence of the key fields.

[UNIQUE|NON-UNIQUE] KEY TABLE LINE

If a table has an elementary line type (C, D, F, I, N, P, T, X), you can define the entire line as the key. If you try this for a table whose line type is itself a table, a syntax error occurs. If a table has a structured line type, it is possible to specify the entire line as the key. However, you should remember that this is often not suitable.

[UNIQUE|NON-UNIQUE] DEFAULT KEY

This declares the fields of the default key as the key fields. If the table has a structured line type, the default key contains all non-numeric columns of the internal table that are not and do not contain references or internal tables. If the table has an elementary line type, the default key is the entire line. The default key of an internal table whose line type is an internal table, the default key is empty.

Specifying a key is optional. If you do not specify a key, the system defines a table type with an arbitrary key. You can only use this to define the types of field symbols and the interface parameters of procedures . For exceptions, refer to Special Features of Standard Tables.

The optional additions UNIQUE or NON-UNIQUE determine whether the key is to be unique or non-unique, that is, whether the table can accept duplicate entries. If you do not specify UNIQUE or NON-UNIQUE for the key, the table type is generic in this respect. As such, it can only be used for specifying types. When you specify the table type simultaneously, you must note the following restrictions:

You cannot use the UNIQUE addition for standard tables. The system always generates the NON-UNIQUE addition automatically.

You must always specify the UNIQUE option when you create a hashed table.

Initial Memory Requirement

You can specify the initial amount of main memory assigned to an internal table object when you define the data type using the following addition:

INITIAL SIZE <n>

This size does not belong to the data type of the internal table, and does not affect the type check. You can use the above addition to reserve memory space for <n> table lines when you declare the table object.

When this initial area is full, the system makes twice as much extra space available up to a limit of 8KB. Further memory areas of 12KB each are then allocated.

You can usually leave it to the system to work out the initial memory requirement. The first time you fill the table, little memory is used. The space occupied, depending on the line width, is 16 <= <n> <= 100.

It only makes sense to specify a concrete value of <n> if you can specify a precise number of table entries when you create the table and need to allocate exactly that amount of memory (exception: Appending table lines to ranked lists). This can be particularly important for deep-structured internal tables where the inner table only has a few entries (less than 5, for example).

To avoid excessive requests for memory, large values of <n> are treated as follows: The largest possible value of <n> is 8KB divided by the length of the line. If you specify a larger value of <n>, the system calculates a new value so that n times the line width is around 12KB.

Examples

TYPES: BEGIN OF LINE,

COLUMN1 TYPE I,

COLUMN2 TYPE I,

COLUMN3 TYPE I,

END OF LINE.

TYPES ITAB TYPE SORTED TABLE OF LINE WITH UNIQUE KEY COLUMN1.

The program defines a table type ITAB. It is a sorted table, with line type of the structure LINE and a unique key of the component COLUMN1.

TYPES VECTOR TYPE HASHED TABLE OF I WITH UNIQUE KEY TABLE LINE.

TYPES: BEGIN OF LINE,

COLUMN1 TYPE I,

COLUMN2 TYPE I,

COLUMN3 TYPE I,

END OF LINE.

TYPES ITAB TYPE SORTED TABLE OF LINE WITH UNIQUE KEY COLUMN1.

TYPES: BEGIN OF DEEPLINE,

FIELD TYPE C,

TABLE1 TYPE VECTOR,

TABLE2 TYPE ITAB,

END OF DEEPLINE.

TYPES DEEPTABLE TYPE STANDARD TABLE OF DEEPLINE

WITH DEFAULT KEY.

The program defines a table type VECTOR with type hashed table, the elementary line type I and a unique key of the entire table line. The second table type is the same as in the previous example. The structure DEEPLINE contains the internal table as a component. The table type DEEPTABLE has the line type DEEPLINE. Therefore, the elements of this internal table are themselves internal tables. The key is the default key - in this case the column FIELD. The key is non-unique, since the table is a standard table.

The table type (and particularly the access method) that you will use depends on how the typical internal table operations will be most frequently executed.

Standard tables

This is the most appropriate type if you are going to address the individual table entries using the index. Index access is the quickest possible access. You should fill a standard table by appending lines (ABAP APPEND statement), and read, modify and delete entries by specifying the index (INDEX option with the relevant ABAP command). The access time for a standard table increases in a linear relationship with the number of table entries. If you need key access, standard tables are particularly useful if you can fill and process the table in separate steps. For example, you could fill the table by appending entries, and then sort it. If you use the binary search option with key access, the response time is logarithmically proportional to the number of table entries.

Sorted tables

This is the most appropriate type if you need a table which is sorted as you fill it. You fill sorted tables using the INSERT statement. Entries are inserted according to the sort sequence defined through the table key. Any illegal entries are recognized as soon as you try to add them to the table. The response time for key access is logarithmically proportional to the number of table entries, since the system always uses a binary search. Sorted tables are particularly useful for partially sequential processing in a LOOP if you specify the beginning of the table key in the WHERE condition.

Hashed tables

This is the most appropriate type for any table where the main operation is key access. You cannot access a hashed table using its index. The response time for key access remains constant, regardless of the number of table entries. Like database tables, hashed tables always have a unique key. Hashed tables are useful if you want to construct and use an internal table which resembles a database table or for processing large amounts of data.