Question

This can be a silly question but I want to be sure 100%.

Is the PK of a DB2 table a clustered index by default?

Answer 1

From: DB2 docs – Clustering indexes

Although a table can have several indexes, only one index can be a clustering index. If you do not define a clustering index for a table, DB2 recognizes the first index that is created on the table as the implicit clustering index when it orders data rows.

So no, by default the Primary Key is NOT the clustered index of the table.

The first created index, unique or not, is the “implicit” clustering index and DB2 tries to insert the records as nearly as possible in the order of the values of this index.

If you later create another index and identify it as clustering, then DB2 identifies it as the clustering index but does not rearrange the data that is already in the table. This can be done with the REORG utility.