Add the correct index

This is a huge topic, but it is also the most important “performance” issue.

The main lesson for a novice is to learn of “composite” indexes. Here’s a quick example:

INDEX(last_name, first_name)

is excellent for these:

WHERE last_name = '...'
WHERE first_name = '...' AND last_name = '...'   -- (order in WHERE does not matter)

but not for

WHERE first_name = '...'   -- order in INDEX _does_ matter
WHERE last_name = '...' OR first_name = '...'   -- "OR" is a killer

Found a mistake? Have a question or improvement idea? Let me know.

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