What columns generally make good indexes?

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 buyersWHERE 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 countryON 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.

Some folks answered a similar question here: How do you know what a good index is?

Basically, it really depends on how you will be querying your data. You want an index that quickly identifies a small subset of your dataset that is relevant to a query. If you never query by datestamp, you don't need an index on it, even if it's mostly unique. If all you do is get events that happened in a certain date range, you definitely want one. In most cases, an index on gender is pointless -- but if all you do is get stats about all males, and separately, about all females, it might be worth your while to create one. Figure out what your query patterns will be, and access to which parameter narrows the search space the most, and that's your best index.

Also consider the kind of index you make -- B-trees are good for most things and allow range queries, but hash indexes get you straight to the point (but don't allow ranges). Other types of indexes have other pros and cons.

Good luck!

It all depends on what queries you expect to ask about the tables. If you ask for all rows with a certain value for column X, you will have to do a full table scan if an index can't be used.

Indexes will be useful if:

• The column or columns have a high degree of uniqueness
• You frequently need to look for a certain value or range of values forthe column.

They will not be useful if:

• You are selecting a large % (>10-20%) of the rows in the table
• The additional space usage is an issue
• You want to maximize insert performance. Every index on a table reduces insert and update performance because they must be updated each time the data changes.

Primary key columns are typically great for indexing because they are unique and are often used to lookup rows.