Doubt in internal tables

Question

hi experts
could you please tell difference between linear search, binary search and hashed mechanism.
above 3 which is faster and why.
if  i use binary search in descending order it will search the records are not could please send why.

Answer 1

Hi Sayeed,
Standard table:
The key access to a standard table uses a sequential search. The time required for an access is linearly dependent on the number of entries in the internal table.
You should usually access a standard table with index operations.
Sorted table:
The table is always stored internally sorted by its key. Key access to a sorted table can therefore use a binary search. If the key is not unique, the entry with the lowest index is accessed. The time required for an access is logarithmically dependent on the number of entries in the internal table.
Index accesses to sorted tables are also allowed. You should usually access a sorted table using its key.
Hash table:
The table is internally managed with a hash procedure. All the entries must have a unique key. The time required for a key access is constant, that is it does not depend on the number of entries in the internal table.
You cannot access a hash table with an index. Accesses must use generic key operations (SORT, LOOP, etc.).
Index table:
The table can be a standard table or a sorted table.
Index access is allowed to such an index table. Index tables can be used to define the type of generic parameters of a FORM (subroutine) or a function module.
Just have a look at these links:
http://help.sap.com/saphelp_nw04/helpdata/en/90/8d7304b1af11d194f600a0c929b3c3/frameset.htm
http://help.sap.com/saphelp_nw04/helpdata/en/74/83015785d811d295a800a0c929b3c3/frameset.htm
Also Go through the following Document
1.1 STANDARD table
Key access to a standard table uses a linear search. This means that the time required for a search is in linear relation to the number of table entries.
You should use index operations to access standard tables.
1.2 SORTED table
Defines the table as one that is always saved correctly sorted.
Key access to a sorted table uses a binary key. If the key is not unique, the system takes the entry with the lowest index. The runtime required for key access is logarithmically related to the number of table entries.
1.3 HASHED table
Defines the table as one that is managed with an internal hash procedure
You can only access a hashed table using the generic key operations or other generic operations ( SORT, LOOP, and so on). Explicit or implicit index operations (such as LOOP ... FROM oe INSERT itab within a LOOP) are not allowed.
1.4 INDEX table
A table that can be accessed using an index.
Index table is only used to specify the type of generic parameters in a FORM or FUNCTION. That means that you can't create a table of type INDEX.
Standard tables and sorted tables are index tables.
1.5 ANY table
Any table is only used to specify the type of generic parameters in a FORM or FUNCTION. That means that you can't create a table of type ANY.
Standard, sorted and hashed  tables belongs to ANY tables.
2. Defining an internal table
DATA itab TYPE table type of line type [WITH UNIQUE/NON-UNIQUE KEY <key>] [Iinitial size n]  [WITH HEADER LINE]
Note: There are also other ways to define an internal table. Please refere to the documentation.
2.1 The KEY option
KEY key1,,keyn :
key1..keyn are fields in the table.  The sequence in which you specify the key is significant.
DEFAULT KEY :
The key fields are the standard keys. Note that you can only specify an empty key for tables with access type STANDARD TABLE.  The standard key basically comprises all tables fields with character-like types (type ( C, STRING, D, T, N, X, XSTRING). In particular, components with a numeric type (also refer to ABAP numeric types) and table components usually do not belong to the standard key.
Example:
types:
  begin of t_makt,
    matnr like makt-matnr,
    maktx like makt-maktx,
  end of t_makt.
data:
Define the table
  gi_makt type sorted table of t_makt with unique key matnr.
Define the work area for the table if necessary
  gi_makt type  t_makt.
3. Reading internal tables
READ TABLE itab WITH TABLE KEY k1 = v1 k2 = v2 [additions]
Note: In case of more than one match, it is the first match that is selected.
STANDARD TABLE: The system searches from the start of the table. The response time is in linear relation to the number of table entries.
     SORTED TABLE: The response time is in logarithmic relation to the number of table entries.
     HASHED TABLE: The response time is constant
READ TABLE itab WITH KEY k1 = v1 k2 = v2  [BINARY SEARCH] [additions]
Note: In case of more than one match, it is the first match that is selected.
STANDARD TABLE: If you use the ... BINARY SEARCH addition, the system uses a binary search. Otherwise, the search is sequential. This assumes that the internal table is sorted in ascending order in the sequence of the specified key fields.
     SORTED TABLE: If the specified key fields form a left-justified extract of the table key, the search is binary, otherwise sequential.
     HASHED TABLE: Sequential search.
READ TABLE itab INDEX i [additions]
Accessing the table entry with the index i.
Additions:
INTO wa - wa is used as output area
     ASSIGNING <fs> - The field symbol <fs> is assigned to the entry. This saves the cost of copying the contents in comparison to the first addition. However, this addition does involve table administration costs, and it is therefore only worthwile for lines longer than around 300 bytes.
     COMPARING f1...fn - If the system find an entry, the system compares the subfields f1, f2, ... with the corresponding fields of the work area before they are transported into it.
     COMPARING ALL FIELDS
     TRANSPORTING f1 f2 - If the system finds an entry, it does not transfer all of the subfields (default) into the work area, but only the specified fields f1 f2 ...; the other subfields remain unchanged.
TRANSPORTING NO FIELDS
Example:
loop at gi_mseg into g_mseg.
   read table gi_makt
     with table key matnr = g_mseg-matnr
     into g_makt.
endloop.
Regards
Sreeni