On May 19, 2011, at 9:53 AM, Robert Haas wrote: > On Wed, May 18, 2011 at 11:00 PM, Greg Smith <g...@2ndquadrant.com> wrote: >> Jim Nasby wrote: >>> I think the challenge there would be how to define the scope of the >>> hot-spot. Is it the last X pages? Last X serial values? Something like >>> correlation? >>> >>> Hmm... it would be interesting if we had average relation access times for >>> each stats bucket on a per-column basis; that would give the planner a >>> better idea of how much IO overhead there would be for a given WHERE clause >> >> You've already given one reasonable first answer to your question here. If >> you defined a usage counter for each histogram bucket, and incremented that >> each time something from it was touched, that could lead to a very rough way >> to determine access distribution. Compute a ratio of the counts in those >> buckets, then have an estimate of the total cached percentage; multiplying >> the two will give you an idea how much of that specific bucket might be in >> memory. It's not perfect, and you need to incorporate some sort of aging >> method to it (probably weighted average based), but the basic idea could >> work. > > Maybe I'm missing something here, but it seems like that would be > nightmarishly slow. Every time you read a tuple, you'd have to look > at every column of the tuple and determine which histogram bucket it > was in (or, presumably, which MCV it is, since those aren't included > in working out the histogram buckets). That seems like it would slow > down a sequential scan by at least 10x.
You definitely couldn't do it real-time. But you might be able to copy the tuple somewhere and have a background process do the analysis. That said, it might be more productive to know what blocks are available in memory and use correlation to guesstimate whether a particular query will need hot or cold blocks. Or perhaps we create a different structure that lets you track the distribution of each column linearly through the table; something more sophisticated than just using correlation.... perhaps something like indicating which stats bucket was most prevalent in each block/range of blocks in a table. That information would allow you to estimate exactly what blocks in the table you're likely to need... -- Jim C. Nasby, Database Architect j...@nasby.net 512.569.9461 (cell) http://jim.nasby.net -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
