"Joshua Tolley" <[EMAIL PROTECTED]> writes: > I've been interested in what it would take to start tracking > cross-column statistics. A review of the mailing lists as linked from > the TODO item on the subject [1] suggests the following concerns: > > 1) What information exactly would be tracked? > 2) How would it be kept from exploding in size? > 3) For which combinations of columns would statistics be kept?
I think then you have 4) How would we form estimates from these stats > The major concern in #1 seemed to be that the most suitable form for > keeping most common value lists, histograms, etc. is in an array, and > at the time of the posts I read, arrays of composite types weren't > possible. This seems much less of a concern now -- perhaps in greatest > part because a test I just did against a recent 8.4devel sure makes it > look like stats on composite type columns aren't even kept. The most > straightforward is that we'd keep a simple multi-dimensional > histogram, but that leads to a discussion of #2. "multi-dimensional histogram" isn't such a simple concept, at least not to me. Histograms aren't a bar chart of equal widths and various heights like I was taught in school. They're actually bars of various widths arranged such that they all of the same heights. It's not clear how to extend that concept into two dimensions. I imagine there's research on this though. What do the GIST statistics functions store? -- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's 24x7 Postgres support! -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
