I have a table that has columns A, B and C. That table will have a lot of rows, let's say a million rows. I have a query that will be executed very often and will search the table by the A column, so I added an index on the A column.
I have another query that will not be executed as often as the first one but still often enough to qualify for a speed increase. That query will search by all the three rows.
My question is, will the second query have any speed increase from the index on column A or would I have to add indexes on all three columns in order to increase read speed? In other words, in a query where I have three WHERE conditions, will MySQL first search by the indexed column? Because if it does, out of those million rows, the first column would filter out maybe 100 or so, and querying those by the other two columns would be somewhat trivial and wouldn't require an index.