Why is more memory required during the delta merge operation?
A delta merge needs about twice the size of the table to perform the merge. Additionally, a temporary working space is needed which is about the size of the largest column of the table.
In case you have a table of 100 GB with 40 GB being the size of largest column, you would need 100 GB * 2 + 40 GB = 240 GB while the merge is executed.
Is the delta merge an online operation?
Most steps of the delta merge are performed online, so concurrent read and write accesses to the table aren't blocked. Only during the following typically short periods of time a lock is acquired blocking change operations:
- Beginning of merge: Move of uncommitted rows from the source delta storage (Delta1) to the target delta storage (Delta2);
- End of merge: Switch from source to target storages