What is use of using HASHED TABLE in sap abap?

Hashed table is useful when your have to work with very big internal table and to read it with
"READ TABLE WITH KEY ..."

The time access is constant !

Definition of a Hashed Table:
"Defines the table as one that is managed with an internal hash procedure. You can imagine a hashed table as a set, whose elements you can address using their unique key. Unlike standard and sorted tables, you cannot access hash tables using an index. All entries in the table must have a unique key.

Access time using the key is constant, regardless of the number of table entries.

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 to INSERT itab within a LOOP) are not allowed."

As long as your records has unique key(s), using hash table will give you a huge performance gain when dealing with large dataset. assuming in your case, 10000 record , and if the key is unique, use hash table. The main use of hash tables is for looking up fixed information from a key. So if you have a report that has personnel number and you want to display their name, you could use a hash table.

Thus:
Code:

types: begin of typ_pernr,
pernr like pa0001-pernr,
ename like pa0001-ename,
end of typ_pernr.
data: ls_pernr type typ_pernr,
lt_pernr type hashed table of typ_pernr with unique key pernr.
...
select pernr ename into table lt_pernr from pa0001.
...
loop at itab.
read table lt_pernr with table key pernr = itab-pernr
into ls_pernr.
write: ls_pernr-ename, itab-data.
endloop.