Solved: data dictionary

Former Member · ‎2007 Jun 21

Hello Experts,

Could you please tell me

1.What is index?

2.What is primary index?

3. What is Secondary index?

4 what is buffering?

5 what are types buffering?

6 Give examples on buffering types?

7 what is check table?

8. what is value table?

9. What is the difference between check table and value table?

10. Please let me know interview questions with answers on data dictionary?

Please do the need full as soon as possible,

I will allot maximum marks

Thanks and regards

Rizwana

Former Member · ‎2007 Jul 06

INDEX

We have to identify unique id from different fields with in the table.

example:suppose the table ia ztest ,our id is as, then we repregented like ztest~as.

PRIMERY KEY

To identify unique record from the table.

FORIGEN KEY.

Main tain the relation between two tables.

ex:suppose you are having two tables zdept and zemp.

the table zdept having the fields dept id dept name . and zemp having the fields emp name and dept name.here two tables having the same field(deptname).

so the table zemp refer to as farigen key table and the key is konw as forigen key.

Former Member · ‎2007 Jun 21

Hi,

http://sapabap.iespana.es/sapabap/manuales/learnabap/

http://help.sap.com/saphelp_nw2004s/helpdata/en/43/41341147041806e10000000a1553f6/frameset.htm

http://help.sap.com/saphelp_nw04/helpdata/en/cf/21eb6e446011d189700000e8322d00/content.htm

http://help.sap.com/saphelp_nw2004s/helpdata/en/cf/21ea31446011d189700000e8322d00/frameset.htm

http://help.sap.com/printdocu/core/Print46c/en/data/pdf/BCDWBDIC/BCDWBDIC.

1,2,3.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

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.

Indexes speed up data selection from the database. They consist of selected fields of a table, of which a copy is then made in sorted order. If you specify the index fields correctly in a condition in the WHERE or HAVING clause, the system only searches part of the index (index range scan).

The system automatically creates the primary index. It consists of the primary key fields of the database table. This means that for each combination of fields in the index, there is a maximum of one line in the table. This kind of index is also known as UNIQUE.

If you cannot use the primary index to determine the result set because, for example, none of the primary index fields occur in the WHERE or HAVINGclauses, the system searches through the entire table (full table scan). For this case, you can create secondary indexes, which can restrict the number of table entries searched to form the result set.

You create secondary indexes using the ABAP Dictionary. There you can create its columns and define it as UNIQUE. However, you should not create secondary indexes to cover all possible combinations of fields.

Only create one if you select data by fields that are not contained in another index, and the performance is very poor. Furthermore, you should only create secondary indexes for database tables from which you mainly read, since indexes have to be updated each time the database table is changed. As a rule, secondary indexes should not contain more than four fields, and you should not have more than five indexes for a single database table.

If a table has more than five indexes, you run the risk of the optimizer choosing the wrong one for a particular operation. For this reason, you should avoid indexes with overlapping contents.

Secondary indexes should contain columns that you use frequently in a selection, and that are as highly selective as possible. The fewer table entries that can be selected by a certain column, the higher that column’s selectivity. Place the most selective fields at the beginning of the index. Your secondary index should be so selective that each index entry corresponds to, at most, five percent of the table entries. If this is not the case, it is not worth creating the index. You should also avoid creating indexes for fields that are not always filled, where their value is initial for most entries in the table.

If all of the columns in the SELECT clause are contained in the index, the system does not have to search the actual table data after reading from the index. If you have a SELECT clause with very few columns, you can improve performance dramatically by including these columns in a secondary index.

What is the difference between primary index and secondary index?

http://help.sap.com/saphelp_47x200/helpdata/en/cf/21eb2d446011d189700000e8322d00/frameset.htm

A difference is made between Primary & Secondary indexes to a table. the primary index consists of the key fields of the table and a pointer to the non-keys-fields of the table. The Primary index is generated automatically when a table is created and is created in the datebase as the same times as the table. It is also possible to define further indexes to a table in the ABAP/4 dictionary, which are then referred to as Secondary indexes.

4,5,6.Whenever an Open SQL statement is used to read a record,

the data buffer is checked first to see whether it is there. If not, the data is read from the database.

If the table's attributes indicate that the data should be buffered,

the record is saved in RAM on the application server in data buffers.

Later, if that record is read again, it is read from the buffer instead of the database.

By buffering data, you increase performance in two important ways:

The programs using the buffered data run faster because they don't have to wait for it to come from the database. This reduces delays waiting for the database and the network that connects it.

The other programs that need to access the database run faster because there is less load on the database and less traffic on the network.

Three types of buffering are possible:

Full buffering

Generic buffering

Single-record buffering

Although you use a check box to indicate the buffering type, only one type can be selected at a time. If you choose more than one, an error message will be displayed.

There are two data buffers on each application server, and the buffering type chosen determines which buffer is used. On an application sever there are:

The generic record buffer

The single-record buffer

The generic record buffer is called TABL (short for "table"). The single-record buffer is TABLP ("P"' stands for "partial").

Full Buffering:

To activate the full buffering type, tickmark the Full check box.

When an attempt is made to read data from a fully buffered table,

the system looks in the TABL buffer for it.

If it is not found there, all rows are loaded from the database into TABL .

This happens whenever a select statement for it is executed, no matter how many records match the where clause.

Even if no records match, all are loaded into the buffer if the table is not already there or

if it has been invalidated due to buffer synchronization.

Loading of TABL does not occur with select single statements; only select/ endselect does this.

If the table is fully buffered and a select single statement is executed, no records are loaded into TABL.

However, if the table is already loaded into TABL, select single will read records from it.

During buffer synchronization, the entire table is invalidated if any record within it changes. The next time data is read the entire table is reloaded.

Full buffering is appropriate for small tables that seldom change.

Generic Buffering:

With generic buffering, a group of records is loaded into TABL instead of the entire table.

To activate this type of buffering, tickmark the Generic check box

and put a number n in the No. of Key Fields so that the n left-most fields of the primary key will group records.

When a record is read and n is 1, all records having the same value

in the first field of the primary key are loaded into the buffer.

When a record is read and n is 2, all records having the same value in the first two fields of the primary key are loaded

into the buffer.

Generic buffering is suitable for tables in which records are usually accessed in sets or groups.

Single Record Buffering:

With this type of buffering, select single picks one record into the single record buffer TABLP.

With this buffering type, records are only buffered when the select single statement is executed.

Select/endselect does not load or read TABLP.

Single-record buffering is appropriate for very large tables in which a small percentage of the records are read often.

For single-record buffering to work, records must be read using the select single statement.

There are two buffers: TABL (the generic record buffer) and TABLP (the single record buffer).

There are also two varieties of the select statement: select and select single. The select statement loads TABL and the select single statement loads TABLP.

When reading, select only reads TABL; it ignores TABLP. select single reads both buffers. This behavior is summarized in Figure 5.20. Remember that a record can only be in one buffer at a time because a table can only have one buffering type.

7,8,9.Value Table

This is maintained at Domain Level.

When ever you create a domain , you can entered allowed values. For example you go to Domain SHKZG - Debit/credit indicator. Here only allowed values is H or S.

When ever you use this Domain, the system will forces you to enter only these values.

This is a sort of master check . To be maintained as a customization object. This mean that if you want to enter values to this table you have to create a development request & transport the same.

Check table

For example you have Employee master table & Employee Transaction table.

When ever an employee Transacts we need to check whether that employee exists , so we can refer to the employee master table.

This is nothing but a Parent & Child relationship . Here data can be maintained at client level , no development involved.

As per DBMS what we call foregin key table, is called as check table in SAP.

There is 1 more important Difference in Value Table and Check Table is that:

1. The contents of the check will be used as an input help(F4 Help) for a particular field on which a check table is assigned.

But the contents of Value Table are never used in Input Help.

The Heirarchy which decides from where to used the Input Help is:

1. Input help defined explicitly in ABAP Program or Dialog Module.

2. Input Help Attached to the referenced Database Table field.

3. Using the contents of Check Table as an input help if neither (1) or (2) help are there.

4. Input help from Fixed value or Value range given in Domain.

Reagrds,

Priyanka.

Former Member · ‎2007 Jun 21

What is Primary key, foreign key ? what is primary index? secondary index?

Ans: Primary index: the primary index contains key fiels of a table and a pointer to non-key fields of the table. The primary index is created automatically when a table is created in database and moreover you can further define reference to the primary index which are known as Secondary index.

Value Table