Explanation
The access to internal tables can be optimized by using the appropriate table type and specifying the table key.
The table key is a set of fields that uniquely identifies a row in the table and determines the sorting order of the table. The table key can be either the primary key or a secondary key. The primary key is defined by the table type and the table definition, while the secondary key is defined by the user using the KEY statement1.
The following results in faster access to internal tables:
B: In a sorted internal table, specifying the primary key completely. A sorted internal table is a table type that maintains a predefined sorting order, which is defined by the primary key in the table definition. The primary key can be either unique or non-unique. A sorted internal table can be accessed using the primary key or the table index. The access using the primary key is faster than the access using the table index, because the system can use a binary search algorithm to find the row. However, the primary key must be specified completely, meaning that all the fields of the primary key must be given in the correct order and without gaps2.
D: In a hashed internal table, specifying the primary key partially from the left without gaps. A hashed internal table is a table type that does not have a predefined sorting order, but uses a hash algorithm to store and access the rows. The primary key of a hashed internal table must be unique and cannot be changed. A hashed internal table can only be accessed using the primary key, not the table index. The access using the primary key is very fast, because the system can directly calculate the position of the row using the hash algorithm. The primary key can be specified partially from the left without gaps, meaning that some of the fields of the primary key can be omitted, as long as they are the rightmost fields and there are no gaps between the specified fields.
E: In a hashed internal table, specifying the primary key completely. A hashed internal table is a table type that does not have a predefined sorting order, but uses a hash algorithm to store and access the rows. The primary key of a hashed internal table must be unique and cannot be changed. A hashed internal table can only be accessed using the primary key, not the table index. The access using the primary key is very fast, because the system can directly calculate the position of the row using the hash algorithm. The primary key can be specified completely, meaning that all the fields of the primary key must be given in the correct order.
The following do not result in faster access to internal tables, because:
A: In a sorted internal table, specifying the primary key partially from the left without gaps. A sorted internal table is a table type that maintains a predefined sorting order, which is defined by the primary key in the table definition. The primary key can be either unique or non-unique. A sorted internal table can be accessed using the primary key or the table index. The access using the primary key is faster than the access using the table index, because the system can use a binary search algorithm to find the row.
However, the primary key must be specified completely, meaning that all the fields of the primary key must be given in the correct order and without gaps. If the primary key is specified partially from the left without gaps, the system cannot use the binary search algorithm and has to perform a linear search, which is slower2.
C: In a standard internal table, specifying the primary key partially from the left without gaps. A standard internal table is a table type that does not have a predefined sorting order, but uses a sequential storage and access of the rows. The primary key of a standard internal table is the standard key, which consists of all the fields of the table row in the order in which they are defined. A standard internal table can be accessed using the primary key or the table index. The access using the primary key is slower than the access using the table index, because the system has to perform a linear search to find the row.
The primary key can be specified partially from the left without gaps, but this does not improve the access speed, because the system still has to perform a linear search.
References: 1: Internal Tables - Overview - ABAP Keyword Documentation 2: Sorted Tables - ABAP Keyword Documentation : Hashed Tables - ABAP Keyword Documentation : Standard Tables - ABAP Keyword Documentation