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Source: aaronfrancis.com
COUNT(*) vs COUNT(id) in MySQL - Aaron Francis
TL;DR: Use COUNT(*), not COUNT(id), when you want to count rows. You have probably read in a bunch of different places that you shouldn't use SELECT(*) in MySQL when you don't need all the data.
While SELECT(*) selects all the columns, COUNT(*) is specifically optimized to count all the rows.
If a secondary index is not present, InnoDB processes SELECT COUNT(*) statements by scanning the clustered index.
It contains the indexed data and a pointer to the primary key (clustered index) where you can get the rest of the data.
While SELECT(*) selects all the columns, COUNT(*) is specifically optimized to count all the rows.
If a secondary index is not present, InnoDB processes SELECT COUNT(*) statements by scanning the clustered index.
It contains the indexed data and a pointer to the primary key (clustered index) where you can get the rest of the data.
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