Is there a TODO anywhere in this discussion? If so, please let me know. --------------------------------------------------------------------------- Mischa Sandberg wrote: > Quoting Mark Lewis <mark.lewis@xxxxxxxx>: > > > If the original paper was published in 1984, then it's been more than > > 20 years. Any potential patents would already have expired, no? > > Don't know, but the idea is pervasive among different vendors ... > perhaps that's a clue. > > And having now read beyond the start of ExecHashJoin(), I can see that > PG does indeed implement Grace hash; and the implementation is nice and > clean. > > If there were room for improvement, (and I didn't see it in the source) > it would be the logic to: > > - swap inner and outer inputs (batches) when the original inner turned > out to be too large for memory, and the corresponding outer did not. If > you implement that anyway (complicates the loops) then it's no trouble > to just hash the smaller of the two, every time; saves some CPU. > > - recursively partition batches where both inner and outer input batch > ends up being too large for memory, too; or where the required number of > batch output buffers alone is too large for working RAM. This is only > for REALLY big inputs. > > Note that you don't need a bad hash function to get skewed batch sizes; > you only need a skew distribution of the values being hashed. > > > > ---------------------------(end of broadcast)--------------------------- > TIP 3: if posting/reading through Usenet, please send an appropriate > subscribe-nomail command to majordomo@xxxxxxxxxxxxxx so that your > message can get through to the mailing list cleanly > -- Bruce Momjian | http://candle.pha.pa.us pgman@xxxxxxxxxxxxxxxx | (610) 359-1001 + If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania 19073 ---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
