Re: Benefits of indexing

[Rick James]

A little longer, ok. A lot longer? Let's see
SHOW CREATE TABLE
EXPLAIN SELECT ...
These may perform terribly:
* ON ABS(a.foo) = ABS(b.foo) -- functions prevent use of indexes
* INDEX(x,y) ... ON a.y = ... -- INDEXes are useful when including first field(s)
* LEFT JOIN -- this forces the order of the tables, preventing some optimizations

The EXPLAIN will have 2 rows -- one for the first table (often the smaller one), and one for the other table. That second row may say "Rows: 1" if it is efficient, and mostly oblivious to the size of the second table. Or the second row may say "type: ALL", in which case it could not use any index.

With the SHOW and EXPLAIN, we can give you more specific guidance.