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1. If the existing secondary key equals to the new secondary key, it is a NO OP.
2. If a different old previous secondary key exists, it deletes the existing secondary key-primary key pair from the secondary index.
3. If a different new secondary key exists, it inserts the new secondary key-primary key pair from the secondary index.
   
   Example:
   data type: [int(pk),int].
   existing data:
   single record {1,5}
   
   a) upsert {1,5}
   In this case, the secondary index upsert the old secondary key == the new secondary key. Hence, it will be no op.
   b) upsert{1,2}
   In this case, the secondary keys are not equal. Hence, we check:
   (previous value exists(not null) which was {5,1}, so we delete it)
   (new value is {2,1} so we insert it).
   c) upsert {2,2}
   In this case, there is no old value, we will only insert the key {2,2} in secondary index.

• The Upsert commit operator commits the transaction creating Upsert logs if an old value was deleted in the transaction and an Insert log otherwise.
• When an Upsert log is flushed to disk, the operation counter is decremented twice since it performed two operations.

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