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Why would excluding records by creating a temporary table of their primary keys be faster than simply excluding by value?
I have two tables with millions of records. Every so often I need to join them and exclude just a handful of records where a bit(1) column is set to 1 instead of 0.
I can do it with either,
WHERE is_excluded !=1
or
WHERE example_table.pk NOT IN
(
SELECT pk FROM(
SELECT pk FROM
example_table
WHERE is_excluded =1)
AS t)
For example
UPDATE example_table
SET textfield = 'X'
WHERE textfield = 'Y'
and pk not in (SELECT pk FROM (SELECT pk FROM example_table WHERE do_not_touch =1)as t) ;
is faster than
UPDATE example_table
SET textfield = 'X'
WHERE textfield = 'Y'
and do_not_touch !=1
The second way is sometimes way faster, even though it takes much longer to write out.
Why would the second way be faster?
2 answers
Why would the second way be faster?
Generally speaking, the first form will perform worse (as well as looking a lot worse) than the second. You are hitting an edge case where the opposite is true, because:
-
The
not inin your first example is likely to be transformed into an anti-join (something like this). Because you also have "…just a handful of records where a bit(1) column is set to 1…" that anti-join is likely to be fairly fast. -
Bad stats or bad luck means that the optimizer is making a wrong choice when filtering. Perhaps it is choosing a full table scan in the second case, or failing to use a good index.
We'd need to know your actual plans/indexes/etc to be able to say more, as several people have mentioned in comments.
0 comment threads
is_excluded = 1 is very different from do_not_touch != 1. Whenever possible, try to structure your data and queries so that you can do an equi-join - that is, compare things using an = comparison. > and < and != can be really bad because the database will at the very least have to do an index scan, if there's an appropriate index available - if not, full table scan, baby! Wooo! If you can use do_not_touch = 0 that would be nice, but I obviously have no idea what that field contains.
Also, it helps to have an appropriate index. For your second query
UPDATE example_table
SET textfield = 'X'
WHERE textfield = 'Y'
and do_not_touch != 1
it would seem that an index on example_table(textfield, do_not_touch) would perhaps be helpful.
1 comment thread