Redologs

 RedoLogs

if an Oracle database is in archivelog mode, (with just one destination) no dataguard enabled.

if you add a register,

in a table without indexes, (as a minimum) this will happen:

one write for the table block,

one write for the undo block of the table,

one write for the redolog entry of the inserted block,

one write for the redolog entry of the undo block

two writes for the archivelog entries of the redolog blocks

  6 IO operations

if you add a register, in a table with one index, and this insert won't trigger an index page split (or more), but it's affected, let's suppose a primary key

(as a minimum) this will happen:

one write for the table block,

one write for the index block

one write for the undo block of the table block

one write for the undo block of the index block

one write for the redolog entry of the table inserted block,

one write for the redolog entry of the index block

one write for the redolog entry of the undo record for the table block

one write for the redolog entry of the undo record for the index block

eight writes for the archivelog entries of the redolog blocks

 16 IO operations just by adding an index

As you can suspect now, more complex tables, with multiple indexes and triggers can easily increment the quantity of IOs that a simple record insert can generate and consider the update or delete, essentially all that we normally do in a database.

The important thing here is to visualize the central role that the redologs have in the database performance tuning