On 24.09.2012 13:57, Amit kapila wrote:
Rebased version of patch based on latest code.
When HOT was designed, we decided that heap_update needs to compare the old and new attributes directly, with memcmp(), to determine whether any of the indexed columns have changed. It was not deemed infeasible to pass down that information from the executor. I don't remember the details of why that was, but you seem to trying to same thing in this patch, and pass the bitmap of modified cols from the executor to heap_update(). I'm pretty sure that won't work, for the same reasons we didn't do it for HOT.
I still feel that it would probably be better to use a generic delta encoding scheme, instead of inventing one. How about VCDIFF (http://tools.ietf.org/html/rfc3284), for example? Or you could reuse the LZ compressor that we already have in the source tree. You can use LZ for delta compression by initializing the history buffer of the algorithm with the old tuple, and then compressing the new tuple as usual. Or you could still use the knowledge of where the attributes begin and end and which attributes were updated, and do the encoding similar to how you did in the patch, but use LZ as the output format. That way the decoding would be the same as LZ decompression.
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