From: Alex P. <pe...@in...> - 2007-01-08 17:02:12
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Dmitry Yemanov: > Alex Peshkov wrote: >> May be I'm a bit out of context here? What histogramms do you mean? > > Value distribution histograms. The same applies to other useful > statistics we need to track -- like avg. record/key length, clustering > factor, etc. Understood. Presence of such histograms may become a really revolutionary step for an optimizer, specially if we learn it to adjust plan according to given parameters' values. > The current selectivity value is not much of help for the > optimizer. > Yes, but having even it unreliable leads to getting bad plans. And having wrong histogram will do it too. Do we already have suggestions concerning format of storing such statistics? If not, it will be good idea to build it taking into an account ability of it's dynamic modification. For histograms this may be even more important then for existing statistics. Imagine an index, which is regularly populated with big portion of records with same key value (but has in average good selectivity). If that value will not get into histogram, optimizer will definitely create bad plans with that index. I'm sure it's possible to have histograms (and all other statistics elements) that are kept dynamically in correct actual state. And though now it's not time to discuss it in all details, it is not a showstopper for dynamical updates in existing statistics. |