Question

Suppose I have table in my DB schema called TEST with fields (id, name, address, phone, comments). Now, I know that I’m going to perform a large set of different queries for that table, therefore my question is next, when and why I shall create indexes like ID_NAME_INDX (index for id and name) and when it’s more efficient to create separately index for id and index for name field(by when I mean for what type of query)?

Answer 1

The general aim would be to “cover” all columns so the query only has to use the index.

-- An index on Name including ID would be ideal
SELECT
    [id]
FROM
    TEST
WHERE
    [name] = 'bob'

Say you need name and indx but have separate indexes. You’ll end up with a bookmark lookup from the index to the PK to get the other columns (assuming it doesn’t just scan the PK)

Edit, after 1st comment:

select * from test where id='id1' and name='Name1'

For this query, the SELECT * but mitigates against any index so the PK would be used.
If you had:

select address from test where id='id1' and name='Name1'

then an index on ID, name including address would “cover” it.

Using “OR” creates difficulties for any strategy. However,

select address from test where id='id1' and name='Name1'

would still use the “ID, name including address” inex most likely but scan it rather that seek

Read this: Execution Plan Basics