On Thu, May 03, 2007 at 01:42:44PM -0700, Andrew Edson wrote: > As the title of this message suggests, I've got a couple of questions about indexing that I'm not sure about. I've tried to take a look at the docs, but I can't remember seeing anything on these; it's quite possible, I admit, that I'm simply not remembering all of what I saw, but I would appreciate it if someone would help me to understand these. > > 1. Does an indexed column on a table have to be a potential primary key? No, that's the difference between unique and non-unique indexes. > The times were almost identical in the following areas: Before Indexing, after Indexing but before Analyzing, and after Analyzing. If you want reasons, you're going to need to provide EXPLAIN ANALYSE output. > I have in mind something like this: > select * from [event table] where substring(cntrct_id, 3,1) = 'H'; > which should select any event records associated with 'cntrct_id' values initally set up in August. (Jan = A, Feb = B, etc) > > If I established an index on the 'cntrct_id' field in the event > tables, would it assist in speeding up the substring-based search, > or would it not be effective at doing so? Not directly, no. However, you can have indexes on expressions: CREATE INDEX foo ON bar((substring(cntrct_id, 3,1))); Which could speed up the above query (could, since it depends on exactly how much of the table actually needs to be searched...) Have a nice day, -- Martijn van Oosterhout <kleptog@xxxxxxxxx> http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to litigate.
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