Optimization about count(*)

[谢立超]

sql:
select count(*) from t where id > 10 and id < 1000000;
In table t, there is an B-tree index on id field, this sql will read
hundreds of thousandes of lines from the storage engine, this is a
time-consuming operation.
If tell the storage engine computing the count in the range, does it
have higher performance? (using records_in_range() api and so on)