Add “Faux” Predicates: Postgres Optimization

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The query fetches sales that were modified before 2019. There is no index on this field, so the optimizer generates an execution plan to scan the entire table.

Let’s say you have another field in this table with the time the sale was created. Since it’s not possible for a sale to be modified before it was created, adding a similar condition on the created field won’t change the result of the query. However, the optimizer might use this information to generate a better execution plan:

db=# (
  SELECT *
  FROM sale
  WHERE modified < '2019-01-01 asia/tel_aviv'
  AND   created < '2019-01-01 asia/tel_aviv';
);
                                           QUERY PLAN
--------------------------------------------------------------------------------------------------
Index Scan using sale_created_ix on sale (cost=0.44..4.52 rows=1 width=276)
  Index Cond: (created < '2019-01-01 00:00:00+02'::timestamp with time zone)
  Filter: (modified < '2019-01-01 00:00:00+02'::timestamp with time zone)

After we added the “Faux Predicate” the optimizer decided to use the index on the created field, and the query got much faster! Note that the previous predicate on the modified field is still being evaluated, but it’s now being applied on much fewer rows.

A “Faux Predicate” should not change the result of the query. It should only be used to provide more information to the optimizer that can improve the query performance. Keep in mind that the database has to evaluate all the predicates, so adding too many might make a query slower.

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