On Dec 13, 2007, at 7:13 AM, Gregory Stark wrote:
We currently execute a lot of joins as Nested Loops which would be more efficient if we could batch together all the outer keys and execute a singleinner bitmap index scan for all of them together.Essentially what I'm saying is that we're missing a trick with Hash Joins which currently require that we can execute the inner side once without anyparameters from the outer side.Instead what we could do is build up the hash table, then scan the hash table building up an array of keys and pass them as a parameter to the inner side. The inner side could do a bitmap index scan to fetch them all at once andstart returning them just as normal to the hash join. There are a couple details:1) Batched hash joins. Actually I think this would be fairly straightforward. You want to rescan the inner side once for each batch. That would actually be easier than what we currently do with saving tuples to files and allthat. 2) How to pass the keys. This could be a bit tricky especially formulti-column keys. My first thought was to build up an actually Array node but that only really works for single-column keys I think. Besides it would be more efficient to somehow arrange to pass over a reference to the wholehash.I fear the real complexity would be (as always) in the planner rather than the executor. I haven't really looked into what it would take to arrange this orhow to decide when to do it.
TODO? -- Decibel!, aka Jim C. Nasby, Database Architect [EMAIL PROTECTED] Give your computer some brain candy! www.distributed.net Team #1828
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