Question

The MySQL query I’m currently trying to perform is functionally equivalent to this:

SELECT small_table.A, small_table.B, small_table.C, huge_table.X, huge_table.Y
FROM small_table LEFT JOIN huge_table
ON small_table.D = huge_table.Z
WHERE small_table.E = 'blah'

except that the query doesn’t appear to terminate (at least not within a reasonable amount of time), probably because the second table is huge (i.e. 7500 rows with a total size of 3 MB). Can I perform a functionally equivalent join in a reasonable amount of time, or do I need to introduce redundancy by adding columns from the huge table into the small table. (I’m a total beginner to SQL.)

The clause WHERE small_table.E = 'blah' is static and 'blah' never changes.

Here is the EXPLAIN output as requested:

Array ( [0] => Array ( [0] => 1 [id] => 1 [1] => SIMPLE [select_type] => SIMPLE [2] => small_table [table] => small_table [3] => ref [type] => ref [4] => E [possible_keys] => E [5] => E [key] => E [6] => 1 [key_len] => 1 [7] => const [ref] => const [8] => 1064 [rows] => 1064 [9] => Using where [Extra] => Using where ) [1] => Array ( [0] => 1 [id] => 1 [1] => SIMPLE [select_type] => SIMPLE [2] => huge_table [table] => huge_table [3] => eq_ref [type] => eq_ref [4] => PRIMARY [possible_keys] => PRIMARY [5] => PRIMARY [key] => PRIMARY [6] => 4 [key_len] => 4 [7] => my_database.small_table.D [ref] => my_database.small_table.D [8] => 1 [rows] => 1 [9] => [Extra] => ) )

Answer 1

A few things …

1) Are you executing this query directly in MySQL (either Workbench GUI or command line), or is this query embedded in PHP code? Your EXPLAIN output seems to suggest PHP. If you haven’t done so already, try executing the query directly in MySQL and take PHP out of the mix.

2) Your EXPLAIN output looks Ok, except I’m wondering about your WHERE clause with small_table.E = 'blah'. The EXPLAIN output shows that there’s an index on column E but the key length = 1, which is not consistent to the comparison with 'blah'. What data type did you use for the column definition for small_table.E?

3) MySQL is estimating that it needs to scan 1064 rows in small_table. How many total rows are in small_table, and how many do you expect should match this particular query?