Question

I have two large tables in MySQL, one containing about 6,00,000 records and another containing about 90,000. I have one common field in these two tables on the basis of which I want to compare the records. I indexed both the tables on this common field but still query execution takes a very long time. Following is the query I used:

SELECT a.url,a.title,a.description,a.jobreferenceno,a.location,a.state,
    a.country,a.created_datetime,a.postalcode,a.company
FROM TABLE1 as a
WHERE EXISTS (
    select b.checkfield
    from TABLE2 as b where a.checkfield=b.checkfield
);

checkfield is the common column in both the tables.

Any suggestions on how to speed it up?

Answer 1

subqueries, like the EXISTS subquery you used here, are notoriously slow in MySQL. You should convert them to a JOIN if you have the chance. A JOIN query for your example would look like this:

SELECT a.url
,a.title
,a.description
,a.jobreferenceno
,a.location
,a.state
,a.country
,a.created_datetime
,a.postalcode
,a.company 
FROM TABLE1 as a 
INNER JOIN TABLE2 as b 
ON         a.checkfield = b.checkfield

(Note that this is not entirely the same as your original: here, any row from TABLE1 for that matches multiple rows in TABLE2 will be returned multiple times. If checkfield is unique in both tables, the result will be the same.)

That said, it is unclear how this really helps – you’re not really comparing rows here, simply selecting those rows from TABLE for which there is at least one row in TABLE2 that happens to have the same value in checkfield

(at any rate, checkfield should be indexed in both tables to help the efficiency of these queries )