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What is ESort? -- An Online External Sort
 
ESort provides a very primitive database API: An insert-only set of keys.
 
There are very different trade-offs when compared with a standard database.

N = number of keys in the set, M = number of operations
The seeks needed to for each repeated operation in one transaction:

                   ESort           BTree            Hash
Insertion          O(1)            O(M*log(N))      O(M)
Read in sequence   O(1)            O(M)             impossible
Seek to key        O(M*log^2(N))   O(M*log(N))      O(M)
Delete             impossible      O(M*log(N))      O(M)

From this table, one can conclude that ESort allows blindly fast insertion
and sequential read, but pays in the cost to seek. This is what makes it
highly appropriate for keyword indexes.

An additional restriction is that ESort only supports a single-writer.
An additional advantage is that ESort readers get snap-shots.