Re: define internal table

Former Member — Thu, 15 Nov 2007 04:34:59 GMT

Internal tables are the core of ABAP. They are like soul of a body. For any program we use

internal tables extensively. We can use Internal tables like normal data base tables only, but the

basic difference is the memory allocated for internal tables is temporary. Once the program is

closed the memory allocated for internal tables will also be out of memory.

But while using the internal tables, there are many performance issues to be considered. i.e which

type of internal table to be used for the program..like standard internal table, hashed internal

table or sorted internal table etc..

Internal tables

Internal tables provide a means of taking data from a fixed structure and storing it in working memory in ABAP. The data is stored line by

line in memory, and each line has the same structure. In ABAP, internal tables fulfill the function of arrays. Since they are dynamic data

objects, they save the programmer the task of dynamic memory management in his or her programs. You should use internal tables

whenever you want to process a dataset with a fixed structure within a program. A particularly important use for internal tables is for

storing and formatting data from a database table within a program. They are also a good way of including very complicated data

structures in an ABAP program.

Like all elements in the ABAP type concept, internal tables can exist both as data types and as data objects A data type is the abstract

description of an internal table, either in a program or centrally in the ABAP Dictionary, that you use to create a concrete data object. The

data type is also an attribute of an existing data object.

Internal Tables as Data Types

Internal tables and structures are the two structured data types in ABAP. The data type of an internal table is fully specified by its line type,

key, and table type.

Line type

The line type of an internal table can be any data type. The data type of an internal table is normally a structure. Each component of the

structure is a column in the internal table. However, the line type may also be elementary or another internal table.

Key

The key identifies table rows. There are two kinds of key for internal tables - the standard key and a user-defined key. You can specify

whether the key should be UNIQUE or NON-UNIQUE. Internal tables with a unique key cannot contain duplicate entries. The uniqueness

depends on the table access method.

If a table has a structured line type, its default key consists of all of its non-numerical columns that are not references or themselves

internal tables. If a 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.

The user-defined key can contain any columns of the internal table that are not references or themselves internal tables. Internal tables

with a user-defined key are called key tables. When you define the key, the sequence of the key fields is significant. You should remember

this, for example, if you intend to sort the table according to the key.

Table type

The table type determines how ABAP will access individual table entries. Internal tables can be divided into three types:

Standard tables have an internal linear index. From a particular size upwards, the indexes of internal tables are administered as trees. In

this case, the index administration overhead increases in logarithmic and not linear relation to the number of lines. The system can access

records either by using the table index or the key. The response time for key access is proportional to the number of entries in the table.

The key of a standard table is always non-unique. You cannot specify a unique key. This means that standard tables can always be filled

very quickly, since the system does not have to check whether there are already existing entries.

Sorted tables are always saved sorted by the key. They also have an internal index. The system can access records either by using the

table index or the key. The response time for key access is logarithmically proportional to the number of table entries, since the system

uses a binary search. The key of a sorted table can be either unique or non-unique. When you define the table, you must specify whether

the key is to be unique or not. Standard tables and sorted tables are known generically as index tables.

Hashed tables have no linear index. You can only access a hashed table using its key. The response time is independent of the number of

table entries, and is constant, since the system access the table entries using a hash algorithm. The key of a hashed table must be unique.

When you define the table, you must specify the key as UNIQUE.

Generic Internal Tables

Unlike other local data types in programs, you do not have to specify the data type of an internal table fully. Instead, you can specify a

generic construction, that is, the key or key and line type of an internal table data type may remain unspecified. You can use generic

internal tables to specify the types of field symbols and the interface parameters of procedures . You cannot use them to declare data

objects.

Internal Tables as Dynamic Data Objects

Data objects that are defined either with the data type of an internal table, or directly as an internal table, are always fully defined in

respect of their line type, key and access method. However, the number of lines is not fixed. Thus internal tables are dynamic data objects,

since they can contain any number of lines of a particular type. The only restriction on the number of lines an internal table may contain are

the limits of your system installation. The maximum memory that can be occupied by an internal table (including its internal administration)

is 2 gigabytes. A more realistic figure is up to 500 megabytes. An additional restriction for hashed tables is that they may not contain more

than 2 million entries. The line types of internal tables can be any ABAP data types - elementary, structured, or internal tables. The

individual lines of an internal table are called table lines or table entries. Each component of a structured line is called a column in the

internal table.

Choosing a Table Type

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.

Creating Internal Tables

Like other elements in the ABAP type concept, you can declare internal tables as abstract data

types in programs or in the ABAP Dictionary, and then use them to define data objects.

Alternatively, you can define them directly as data objects. When you create an internal table as a

data object, you should ensure that only the administration entry which belongs to an internal

table is declared statically. The minimum size of an internal table is 256 bytes. This is important if an

internal table occurs as a component of an aggregated data object, since even empty internal

tables within tables can lead to high memory usage. (In the next functional release, the size of the

table header for an initial table will be reduced to 8 bytes). Unlike all other ABAP data objects, you

do not have to specify the memory required for an internal table. Table rows are added to and

deleted from the table dynamically at runtime by the various statements for adding and deleting

records.

You can create internal tables in different types.

You can create standard internal table and then make it sort in side the program.

The same way you can change to hashed internal tables also.

There will be some performance issues with regard to standard internal tables/ hashed internal

tables/ sorted internal tables.

Internal table types

This section describes how to define internal tables locally in a program. You can also define internal tables globally as data types in the

ABAP Dictionary.

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.

Internal table objects

Internal tables are dynamic variable data objects. Like all variables, you declare them using the DATA statement. You can also declare static

internal tables in procedures using the STATICS statement, and static internal tables in classes using the CLASS-DATA statement. This

description is restricted to the DATA statement. However, it applies equally to the STATICS and CLASS-DATA statements.

Reference to Declared Internal Table Types

Like all other data objects, you can declare internal table objects using the LIKE or TYPE addition of the DATA statement.

DATA <itab> TYPE <type>|LIKE <obj> [WITH HEADER LINE].

Here, the LIKE addition refers to an existing table object in the same program. The TYPE addition can refer to an internal type in the

program declared using the TYPES statement, or a table type in the ABAP Dictionary.

You must ensure that you only refer to tables that are fully typed. Referring to generic table types (ANY TABLE, INDEX TABLE) or not

specifying the key fully is not allowed (for exceptions, refer to Special Features of Standard Tables).

The optional addition WITH HEADER line declares an extra data object with the same name and line type as the internal table. This data

object is known as the header line of the internal table. You use it as a work area when working with the internal table (see Using the

Header Line as a Work Area). When you use internal tables with header lines, you must remember that the header line and the body of the

table have the same name. If you have an internal table with header line and you want to address the body of the table, you must indicate

this by placing brackets after the table name (<itab>[]). Otherwise, ABAP interprets the name as the name of the header line and not of the

body of the table. You can avoid this potential confusion by using internal tables without header lines. In particular, internal tables nested

in structures or other internal tables must not have a header line, since this can lead to ambiguous expressions.

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

DATA: ITAB TYPE VECTOR,

JTAB LIKE ITAB WITH HEADER LINE.

MOVE ITAB TO JTAB. <- Syntax error!

MOVE ITAB TO JTAB[].

The table object ITAB is created with reference to the table type VECTOR. The table object JTAB has the same data type as ITAB. JTAB also

has a header line. In the first MOVE statement, JTAB addresses the header line. Since this has the data type I, and the table type of ITAB

cannot be converted into an elementary type, the MOVE statement causes a syntax error. The second MOVE statement is correct, since

both operands are table objects.

Declaring New Internal Tables

You can use the DATA statement to construct new internal tables as well as using the LIKE or TYPE addition to refer to existing types or

objects. The table type that you construct does not exist in its own right; instead, it is only an attribute of the table object. You can refer to

it using the LIKE addition, but not using TYPE. The syntax for constructing a table object in the DATA statement is similar to that for defining

a table type in the TYPES statement.

DATA <itab> TYPE|LIKE <tabkind> OF <linetype> WITH <key>

[INITIAL SIZE <n>]

[WITH HEADER LINE].

As when you define a table type , the type constructor

defines the table type <tabkind>, the line type <linekind>, and the key <key> of the internal table <itab>. Since the technical attributes of

data objects are always fully specified, the table must be fully specified in the DATA statement. You cannot create generic table types (ANY

TABLE, INDEX TABLE), only fully-typed tables (STANDARD TABLE, SORTED TABLE, HASHED TABLE). You must also specify the key and whether

it is to be unique (for exceptions, refer to Special Features of Standard Tables).

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

You can create an internal table with a header line using the WITH HEADER LINE addition. The header line is created under the same

conditions as apply when you refer to an existing table type.

DATA ITAB TYPE HASHED TABLE OF SPFLI

WITH UNIQUE KEY CARRID CONNID.

The table object ITAB has the type hashed table, a line type corresponding to the flat structure SPFLI from the ABAP Dictionary, and a

unique key with the key fields CARRID and CONNID. The internal table ITAB can be regarded as an internal template for the database table

SPFLI. It is therefore particularly suitable for working with data from this database table as long as you only access it using the key.

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