Database indexes - does the table size matter?

What I mean is: Does a table with 20 columns benefit more from indexing a certain field (one that's used in search-ish queries) than a table that has just 4 columns?

Also: What is the harm in adding index to fields that I don't search with much, but might later in the future? Is there a negative to adding indexes? Is it just the size it takes up on disk, or can it make things run slower to a开发者_如何转开发dd unnecessary indexes?

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I'm using Postgres (latest version) and I have one table that I'll be doing a lot of LIKE type queries, etc but the values will undoubtedly change often since my clients have access to CRUD. Should I can the idea of indexes? Are they just a headache?

Does a table with 20 columns benefit more from indexing a certain field (one that's used in search-ish queries) than a table that has just 4 columns?

No, number of columns in a table has no bearing on benefits from having an index.

An index is solely on the values in the column(s) specified; it's the frequency of the values that will impact how much benefit your queries will see. For example, a column containing a boolean value is a poor choice for indexing, because it's a 50/50 chance the value will be one or the other value. At a 50/50 split over all the rows, the index doesn't narrow the search for a particular row.

What is the harm in adding index to fields that I don't search with much, but might later in the future?

Indexes only speed up data retrieval when they can be used, but they negatively impact the speed of INSERT/UPDATE/DELETE statements. Indexes also require maintenance to keep their value.

If you are doing LIKE queries you may find that indexes are not not much help anyway. While an index might improve this query ...

select * from t23
where whatever like 'SOMETHING%'
/

... it is unlikely that an index will help with either of these queries ...

select * from t23
where whatever like '%SOMETHING%'
/

select * from t23
where whatever like '%SOMETHING'
/

If you have free text fields and your users need fuzzy matching then you should look at Postgres's full text functionality. This employs the MATCH operator rather than LIKE and which requires a special index type. Find out more.

There is a gotcha, which is that full text indexes are more complicated than normal ones, and the related design decisions are not simple. Also some implementations require additional maintenance activities.