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Comments on Why would excluding records by creating a temporary table of their primary keys be faster than simply excluding by value?

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Why would excluding records by creating a temporary table of their primary keys be faster than simply excluding by value?

+11
−1

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?

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General comments (7 comments)
Post
+4
−0

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.

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General comments (1 comment)
General comments
Jack Douglas‭ wrote about 4 years ago · edited about 4 years ago

An index on textfield, do_not_touch will be less efficient than one on just textfield because the values in do_not_touch are heavily skewed — adding it to the index just means more IO when scaning.