I had some trouble figuring out why http://localhost, was responding slow compared to 127.0.0.1 when accessing my local apache http server. It turned out that by default the hosts file contains the following line:
# 127.0.0.1 localhost
By uncommenting this line, as follows (remove the hash):
127.0.0.1 localhost
A DNS look-up will then never be performed before it is determined that localhost is actually 127.0.0.1.
Sometimes a parent table have more than one table with multiple references to a field in the parent table. Then the question is how to count references from more than one table in a single query. I have found 5 different ways to achieve this result, and therefore I have run a few tests, to see which was better.
To test this I made this small PHP script, that creates a parent table and child table. Then a loop populates the tables with 1,000 parent rows, approximately 15 children of type 1 and approximately 75 children of type 2. This is done before each query, to make sure that all queries have the same conditions.
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