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PK can a single Field or a combination of fields, that totally depends how any record in the table can be uniquely identified in the table.
By default, Primary Index is created on the Primary Key, but you can create a secondary indexes on other fields and I believe there is no limit on that.
Index of a table is like the Index of your book where the First page on any topic is stored and you can jump directly to that page only, without going through the pages/topics before your the topic that you are looking.
Just think for a 1000 page book without any index......everytime you have to go through each & every page to search until you don't get the topic of your interest.
Similary, the Table index stores the Memory address of the Key/Field on which it is created and hence when you mage your access to the table based on that Table key/field..Index helps the runtime enviroment to read the Record quickly.
Primary Index stores the memory address of all the records of the table along with the primary key value which uniquely identifies the record ....Similarly Secondary index stores the Memory address and the value of each record of the table.
As I mentioned there is no limit on the no of Secondary Indexes. You can create as many as you want.
There will be some tables with larger data which you will be accessing frequently based on a non key field like a Accounting Document Number Table and non key field can be Document Type, in such cases you create Secondary indexes so that system should start using that.
You can use Txn ST05 to check whether Secondary indexed is being used or not in a database fetch.
Secondary index if not used properly can be a overhead for the system and hence use with a precaution.
Bluesky
