Right field for index

Question

Hello
F1 is the only key field of big transparent table T.
I use the following SELECT FROM :
SELECT  *
FROM T
WHERE F1 = XX AND F5 = YY
Should I create the index that includes both fields (F1 , F5) or just F5?
How does the system choose indices during selection operation?
thanks

Answer 1

F1 is the only key field of big transparent table T.
Not clear, because this could mean big as in many rows or long data rows or both...
How does the system choose indices during selection operation?
That's a long story. If you're interested you should do some background reading, a start might be [Wikipedia's database entry|http://en.wikipedia.org/wiki/Database#Indexing], but there's plenty others (if you're using Oracle I strongly recommend looking at the Oracle documentation).
I use the following SELECT FROM :
SELECT * FROM T WHERE F1 = XX AND F5 = YY
Should I create the index that includes both fields (F1 , F5) or just F5?
Due to lack of information obviously nobody will be able to answer your question. And once you actually started gathering the details, the answer should become more obvious to you...
Let's assume that your field F1 is very selective (i.e. specifying a value will return just few rows), then an additional index which contains fields F1/F5 is not expected to have much benefit. After all, we like to avoid defining additional indexes if possible, because it means additional overhead during updates (indexes have to be maintained) and it also makes the life of the database optimizer harder (more choices to pick from).
Now, if the result set of your query is huge and returns most table rows, then it might actually even be better to do a full table scan and not to look at any index at all (simple reason is that an index adds another level of indirection, i.e. the database first has to identify the index rows and then look up for those rows the actual data from the table).
And finally what about comparing frequency of database updates on your tables versus frequency of running your one specific query, space considerations (additional index needs more space), etc. One should make sure to consider all those things and in the end you can probably give the answer yourself...
Cheers, harald