I need to update several columns in one table, based on columns in another. To start with I am just updating one of them. I have tried 2 ways of doing this, which both work, but they are taking about 4 minutes using mySQL commands, and over 20 when run in php. Both tables are about 20,000 rows long.
My question is, is there a better or more efficient way of doing this?
Method 1:
UPDATE table_a,table_b
SET table_a.price = table_b.price
WHERE table_a.product_code=table_b.product_code
Method 2:
UPDATE table_a INNER JOIN table_b
ON table_a.product_code = table_b.product_code
SET table_a.price=table_b.price
I guess that these basically work in the same way, but I thought that the join would be more efficient. The product_code column is random text, albeit unique and every row matches one in the other table.
Anything else I can try?
Thanks
UPDATE: This was resolved by creating an index eg
CREATE UNIQUE INDEX index_code on table_a (product_code)
CREATE UNIQUE INDEX index_code on table_b (product_code)
If your queries are running slowly you'll have to examine the data that query is using.
Your query looks like this:
UPDATE table_a INNER JOIN table_b
ON table_a.product_code = table_b.product_code
SET table_a.price=table_b.price
In order to see where the delay is you can do
EXPLAIN SELECT a.price, b.price FROM table_b b
INNER JOIN table_a a ON (a.product_code = b.product_code)
This will tell you if indexes are being used, see the info on EXPLAIN and more info here .
In your case you don't have any indexes ( possible keys = null
) forcing MySQL to do a full table scan.
You should always do an explain select
on your queries when slowness is an issue. You'll have to convert non-select queries to a select
, but that's not difficult, just list all the changed fields in the select clause and copy join
and where
clauses over as is.
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