Question

Schema:

CREATE TABLE [Groups] ([Group] VARCHAR(50) NULL, [ArtNum] INTEGER NULL, [ID] VARCHAR(50)  NULL, [Time] INTEGER);
CREATE INDEX [GroupTime]   ON [Groups] ([Group], [Time] DESC);
CREATE INDEX [GroupArtNum] ON [Groups] ([Group], [ArtNum]);

I would like optimize the following query:

 SELECT `ID` FROM `Groups` WHERE `Group`=? ORDER BY `Time` DESC LIMIT 1

The problem is that SQLite insists on using the GroupArtNum index instead of the GroupTime index. EXPLAIN QUERY PLAN prints the following:

SEARCH TABLE Groups USING INDEX GroupArtNum (Group=?) (~10 rows)
USE TEMP B-TREE FOR ORDER BY

What’s interesting is that if I try to sort by ArtNum instead of Time, it runs very quickly.

I have only (64-bit) integers in the Time column, same as with ArtNum.

Answer 1

1. Run ANALYZE (which might or might not help in any specific case); or

2. use the INDEXED clause to force using a specific index (although the documentation warns against doing this):

   SELECT ID FROM Groups INDEXED BY GroupTime WHERE ...
   

3. or update to SQLite 3.7.15 (when it’s released), which has query planner optimizations that make it recognize that using GroupTime is more efficient.