Re: Database storage bloat

"Tony and Bryn Reina" <reina_ga(at)hotmail(dot)com> writes:
> CREATE TABLE SegmentValues (
> dbIndex integer REFERENCES EntityFile (dbIndex),
> dwunitid smallint,
> dwsampleindex smallint,
> dtimestamp float4,
> dvalue float4,
> PRIMARY KEY (dbIndex, dtimestamp, dwsampleindex, dwunitid)
> );

> I suppose one thing the binary flat file may be doing is not including the
> time stamp in table SegmentValues. Since I know the sampling rate, I can
> just calculate the timestamp on the fly by the rate times the index
> (assuming no time offset). That would lose a float4 field, but would add
> back a smallint field to the table.

That won't buy you anything at all --- the two bytes saved would be lost
again to alignment padding. (I'm assuming you're on PC hardware with
MAXALIGN = 4 bytes.)

I don't see orders-of-magnitude bloat here though. You've got 16 bytes
of useful data per row (which I suppose was 12 bytes in the flat file?).
There will be 28 bytes of overhead per table row. In addition the index
will require 12 data bytes + 12 overhead bytes per entry; allowing for
the fact that b-tree only likes to pack pages about 2/3ds full, we could
estimate index size as about 36 bytes per original row, giving an
aggregate bloat factor of 6.67X compared to a binary flat file if the
flat file needed 12 bytes per row.

The only way I could see to get to a 65X bloat factor would be if you'd
repeatedly updated the table rows without vacuuming.

regards, tom lane