SQL INDEX not used on WHERE ABS(x-y) < k condition, but used on y - k < x < y + k condition

Question

I have a query involving couples of rows which have a less-than-2-hours time-difference (~0.08333 days):

SELECT mt1.*, mt2.* FROM mytable mt1, mytable mt2 
          


        

Answer 1

I believe that the inclusion of AND is the reasoning as per :

The WHERE clause on a query is broken up into "terms" where each term is separated from the others by an AND operator. If the WHERE clause is composed of constraints separate by the OR operator then the entire clause is considered to be a single "term" to which the OR-clause optimization is applied.

The SQLite Query Optimizer Overview

It may be worthwhile running ANALYZE to see if that improves matters.

As per the comment:

I think the previously added paragraph can clarify why ABS(x-y) < k is not using index, and why x < y + k is using it, don't you think so? Would you want to include this paragraph? [All terms of the WHERE clause are analyzed to see if they can be satisfied using indices. To be usable by an index a term must be of one of the following forms: column = expression, column IS expression, column > expression ...

The following has been added.

To be usable by an index a term must be of one of the following forms:
column = expression
column IS expression
column > expression
column >= expression
column < expression
column <= expression
expression = column
expression > column
expression >= column
expression < column
expression <= column
column IN (expression-list)
column IN (subquery)
column IS NULL

I'm not sure if it would work with a BETWEEN (e.g. WHERE column BETWEEN expr1 AND expr2).

Answer 2

You are using an expression index. The documentation says:

The SQLite query planner will consider using an index on an expression when the expression that is indexed appears in the WHERE clause or in the ORDER BY clause of a query, exactly as it is written in the CREATE INDEX statement. The query planner does not do algebra.

So it is not possible to use an index to speed up lookups of a call to abs() if the indexed expression is only a parameter. (And it is not possible to index the entire abs() call because it involves two tables.)

So converting the expression as you did is the only way to make it more efficient.

(Please note that a<b<c compares a and b first, and then compares the resulting boolean value against c. This is not what you want.)