Re: TOAST compression

[Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>]

Neil Conway wrote:
> toast_compress_datum() considers compression to be "successful" if the
> compressed version of the datum is smaller than the uncompressed
> version. I think this is overly generous: if compression reduces the
> size of the datum by, say, 0.01%, it is likely a net loss to use the
> compressed version of the datum since we'll need to pay for LZ
> decompression every time that we de-TOAST it. This situation can occur
> frequently when storing "mostly-uncompressible" data (compressed images,
> encrypted data, etc.) -- some parts of the data will compress well (e.g.
> metadata), but the vast majority will not.
> 
> It's true that LZ decompression is fast, so we should probably use the
> compressed version of the datum unless the reduction in size is very
> small. I'm not sure precisely what that threshold should be, however.
> 
> Comments?

20%?  25%

-- 
  Bruce Momjian   http://candle.pha.pa.us
  SRA OSS, Inc.   http://www.sraoss.com

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