Comments on: Database Indexes on Domain Names

By: Joseph Scott

Joseph Scott — Wed, 24 Feb 2010 16:09:52 +0000

There definitely more tests that could be done in this area, but I think you’ll find the same general rule of LIKE ‘%blahblahblah’ being slower than LIKE ‘blahblahblah%’ will apply.

By: Angelo Mandato

Angelo Mandato — Wed, 24 Feb 2010 13:55:32 +0000

I believe only the first X characters in a VARCHAR are indexed, which explains why you may find that your query is more efficient reversed like it is. I would recommend doing some testing with very large domain names and many more records. Don’t forget the MySQL server’s index cache size is a factor, if the table has way more VARCHAR indexes than the cache can store then it will still do a full scan.

By: Joseph Scott

Joseph Scott — Wed, 24 Feb 2010 06:43:19 +0000

I’d expect a full index scan to be faster than a full table scan in most cases, but yes, it will still be noticeably slower than an index lookup.

PostgreSQL supports functional indexes – http://www.postgresql.org/docs/8.4/static/sql-createindex.html – by allowing you to include an expression in CREATE INDEX.

By: Artem Russakovskii

Artem Russakovskii — Wed, 24 Feb 2010 06:39:14 +0000

Yeah, either way all would be slow unless you use this solution or go more advanced.

As far as I remember, PostgreSQL supports functional indexes and MySQL definitely doesn’t.

By: Joseph Scott

Joseph Scott — Wed, 24 Feb 2010 06:35:32 +0000

I’d need to go back and review query plans for different database systems, but I believe in some they will use a the index. Now when I say “use the index” it’s often not in the way you think, they will do a full index scan. That’s not the same as doing an index lookup.

MySQL indexes are fairly primitive. For instance as far as I know there’s no support for functional indexes.

By: Artem Russakovskii

Artem Russakovskii — Wed, 24 Feb 2010 06:25:30 +0000

That’s a pretty good idea, though it’s an old trick in the book. But tell me, what database would use an index for a ‘%foo’ query?

As far as I know, no index supports that and the only way to do that would be to go with either FULLTEXT search (in which case you’re stuck with MyISAM and the query would look different) or go for a separate solution, such as Solr or sphinx.