| From: | Nicholas White <n(dot)j(dot)white(at)gmail(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Troels Nielsen <bn(dot)troels(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Request for Patch Feedback: Lag & Lead Window Functions Can Ignore Nulls |
| Date: | 2013-06-28 00:52:34 |
| Message-ID: | CA+=vxNbwaZHZYrDcGR0uhmLTJW0jV7by2tPjKvrVvPCZXZifPA@mail.gmail.com |
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| Lists: | pgsql-hackers |
> The result of the current patch using lead
Actually, I think I agree with you (and, FWIW, so does Oracle:
http://docs.oracle.com/cd/E11882_01/server.112/e25554/analysis.htm#autoId18).
I've refactored the window function's implementation so that (e.g.) lead(5)
means the 5th non-null value away in front of the current row (the previous
implementation was the last non-null value returned if the 5th rows in
front was null). These semantics are slower, as the require the function to
scan through the tuples discarding non-null ones. I've made the
implementation use a bitmap in the partition context to cache whether or
not a given tuple produces a null. This seems correct (it passes the
regression tests) but as it stores row offsets (which are int64s) I was
careful not to use bitmap methods that use ints to refer to set members.
I've added more explanation in the code's comments. Thanks -
| Attachment | Content-Type | Size |
|---|---|---|
| lead-lag-ignore-nulls.patch | application/octet-stream | 29.8 KB |
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