If statistics are not available for a column in a join, the optimizer uses default values:
Operator type |
Examples |
Default selectivity |
|---|---|---|
Equality |
t1.c1 = t1.c2 |
1/rows in smaller table |
Nonequality |
t1.c1 > t1.c2 t1.c1 >= t1.c2 t1.c1 < t1.c2 t1.c1 <= t1.c2 |
33% |
For example, in the following query, the optimizer uses 1/500 for the join selectivity for both tables if there are no statistics for either city column, and stores has 500 rows and authors has 5000 rows:
select au_fname, au_lname, stor_name
from authors a, stores s
where a.city = s.city
