Nico, I respectfully disagree, if you look at my first post you can see that the first query does consider that single value index on s covering. Indeed all the indexes here have all the required data to be covering for their queries.
As David says, it seems there is a missed optimization opportunity here. When looking at the EXPLAIN output, the non-covering index version does not seem to actually use the table value it is copying, but I'm having a hard time decyphering it. On large tables, this is the difference between a 4ms search and a 50ms search… On Wed, Aug 9, 2017, 9:29 PM Nico Williams <n...@cryptonector.com> wrote: > On Wed, Aug 09, 2017 at 06:59:18PM +0000, Wout Mertens wrote: > > but… index s is covering and only includes the field s? I thought a > > covering index was one where all the data needed to satisfy the query is > in > > index? I would say that all the indexes here conform to that definition? > > No, "covering" means that the columns listed in the index include all > the columns from the source table that you need for a given query: > > CREATE TABLE t(j TEXT, s TEXT, foo TEXT); > SELECT s, j FROM t WHERE s = 'foo'; -- full table scan bc [s] is not > -- indexed, is not a PRIMARY KEY, > -- and is not UNIQUE > CREATE INDEX t1 ON t(s); > SELECT s FROM t WHERE s = 'foo'; -- uses index; index covers column > -- selection (just [s]) > > SELECT s, j FROM t WHERE s = 'foo'; -- full table scan unless [s] is > -- a PRIMARY KEY > > CREATE INDEX t2 ON t(j, s); > SELECT s, j FROM t WHERE s = 'foo'; -- full table scan; t2 doesn't > -- help because we need a covering > -- index where [s] is a prefix > > CREATE INDEX t3 ON t(s, j); > SELECT s, j FROM t WHERE s = 'foo'; -- uses covering index t3, finally > > SELECT s, j, foo FROM t WHERE s = 'foo'; -- t3 does not cover -> full > -- table scan > > Usually you should have a PRIMARY KEY, and if [s] were one here, then > none of these would need full table scans, but only two of these would > use only an index and not also index the table via the PK. > > -- truly covering index, but only usable in queries by [s] or [s], > -- [j], or [s], [j], [foo]: > CREATE INDEX t4 ON t(s, j, foo); > > Nico > -- > _______________________________________________ > sqlite-users mailing list > sqlite-users@mailinglists.sqlite.org > http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users > _______________________________________________ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
