One of our customer wrote when I asked him why he had created a combination index on the columns that already had individual indexes and he wrote...
index 1 - res_entity_type
index 2 - res_level
index 3 - res_entity_type, res_level
index4 - res_status, res_int_ext8, res_entity_type

What I am concerned about is index 3 !

Part1:
------
In our mwebres table, the res_entity_type column values are very skewed. We have 72 rows where
res_entity_type = 1 and 32,456 rows where res_entity_type = 3. I added a histogram on res_entity_type so that when the query asks for rows where res_entity_type = 1, at least the optimizer will use the index for that query.
If the query asks for rows where res_entity_type = 3, it should never use an index if the query is just based on that column. It would be more efficient to do the full table scan.

--I know that the 1st part is correct.
Part2:
------
That is why I added indexes that include that column
along with others. For example, for queries that request rows where res_entity_type = 3 and where res_level = 5 (which has 18 rows in our database), the optimizer will go ahead and use an index that contains those two columns.

What I am confused is the 2nd Part. Is what he saying true/correct ?

thanks
Sonali