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I would like to understand how exactly does sql count work. Is it a whole table scan that happens or is it some property of the table that is read. However I feel a table scan would be an overhead in case of huge tables with lots of records.
In general either a table or index scan is performed. This is chiefly because in a MVCC-supporting engine, different transactions could see different rows, so there is no single “row count” which is simultaneously correct for everyone.
Likewise, if you have a WHERE clause, then the where condition could be different for different clients, so they see different numbers.
If you need to do a lot of counts of large tables, consider storing your own counters in a different table. Exactly how you do this is entirely application specific.