What columns generally make good indexes?

Question

As a follow up to "What are indexes and how can I use them to optimise queries in my database?" where I am attempting to learn about indexes, what columns are good index candidates? Specifically for an MS SQL database?

After some googling, everything I have read suggests that columns that are generally increasing and unique make a good index (things like MySQL's auto_increment), I understand this, but I am using MS SQL and I am using GUIDs for primary keys, so it seems that indexes would not benefit GUID columns...

Answer 1

Indexes can play an important role in query optimization and searching the results speedily from tables. So it is most important step to select which columns to be indexed. There are two major places where we can consider indexing: columns referenced in the WHERE clause and columns used in JOIN clauses. In short, such columns should be indexed against which you are required to search particular records. Suppose, we have a table named buyers where the SELECT query uses indexes like below:

SELECT
 buyer_id /* no need to index */
FROM buyers
WHERE first_name='Tariq' /* consider to use index */
AND last_name='Iqbal'   /* consider to use index */

Since "buyer_id" is referenced in the SELECT portion, MySQL will not use it to limit the chosen rows. Hence, there is no great need to index it. The below is another example little different from the above one:

SELECT
 buyers.buyer_id, /* no need to index */
 country.name    /* no need to index */
FROM buyers LEFT JOIN country
ON buyers.country_id=country.country_id /* consider to use index */
WHERE
 first_name='Tariq' /* consider to use index */
AND
 last_name='Iqbal' /* consider to use index */

According to the above queries first_name, last_name columns can be indexed as they are located in the WHERE clause. Also an additional field, country_id from country table, can be considered for indexing because it is in a JOIN clause. So indexing can be considered on every field in the WHERE clause or a JOIN clause.

The following list also offers a few tips that you should always keep in mind when intend to create indexes into your tables:

• Only index those columns that are required in WHERE and ORDER BY clauses. Indexing columns in abundance will result in some disadvantages.
• Try to take benefit of "index prefix" or "multi-columns index" feature of MySQL. If you create an index such as INDEX(first_name, last_name), don’t create INDEX(first_name). However, "index prefix" or "multi-columns index" is not recommended in all search cases.
• Use the NOT NULL attribute for those columns in which you consider the indexing, so that NULL values will never be stored.
• Use the --log-long-format option to log queries that aren’t using indexes. In this way, you can examine this log file and adjust your queries accordingly.
• The EXPLAIN statement helps you to reveal that how MySQL will execute a query. It shows how and in what order tables are joined. This can be much useful for determining how to write optimized queries, and whether the columns are needed to be indexed.

Update (23 Feb'15):

Any index (good/bad) increases insert and update time.

Depending on your indexes (number of indexes and type), result is searched. If your search time is gonna increase because of index then that's bad index.

Likely in any book, "Index Page" could have chapter start page, topic page number starts, also sub topic page starts. Some clarification in Index page helps but more detailed index might confuse you or scare you. Indexes are also having memory.

Index selection should be wise. Keep in mind not all columns would require index.