Question

i got questions about indexing SQL database:

1. Is it better to index boolean column or rather not because there are only 2 options? i know if the table is small then indexing will not change anything, but im asking about table with 1mln records.
2. If i got two dates ValidFrom and ValidTo is it better to create 1 index with 2 columns or 2 seperate indexes? In 90% of queries i use where validfrom < date && validto > date, but there are also few selects only with validfrom or only with validto
3. whats the diffrence between clustered and non-clistered index? i cant find any article, so a link would be great

Answer 1

You both tagged MySQL and SQL-server. This answer is MySQL inspired.

1. It depends on many things, but more important than the size is the variation. If about 50% of the values are TRUE, that means the rest of the values (also about 50%) are FALSE and an index will not help much. If only 2% of the values are TRUE and your queries often only need TRUE records, this index will be useful!
2. If your queries often use both, put both in the index. If one is used more than the other, put that one FIRST in the index, so the composite index can be used for the one field as well.
3. A clustered index means that the data actually is inside the index. A non-clustered index just points to the data, which is actually stored elsewhere. The PRIMARY KEY in InnoDB is a clustered index.

If you want to use Indexes in MySQL, EXPLAIN is your friend!