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
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
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
> > 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);
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