>>> Calculation
>> >varchar = (overhead) 4 + (actual length of string) 3 = 7 bytes
>> >*for 5 varchar cols =
>> >5*7 = 35 bytes
>> >
>> >numeric (according to manual--- The actual storage requirement is
>> >two bytes for each group of four decimal digits, plus eight bytes
>>> overhead )
>>> numeric = ( 10/4)*2 +8 = 13 bytes
>>> *for 5 numeric cols = 13 *5 = 65
> >Not that it will make much difference, but you need to round up in
> >the 10/4 part so you get 14 bytes, not 13.
>Also, this calculation is ignoring the fact that (pre-8.3) varlena
>values have to be int-aligned, so there's wasted pad space too.
>The varchar values really need 8 bytes each, and the numeric values
>16, so the actual data payload in each row is 120 bytes. Then add
>HeapTupleHeader (28 bytes), and then MAXALIGN the whole row size.
>So the rowsize would be either 148 or 152 bytes depending on if you
>were on a machine with 8-byte MAXALIGN. Then add the per-row item
>pointer, giving total per-row space of 152 or 156 bytes. That
>means you can fit either 53 or 52 rows per page, giving either 188
>or 192 pages as the minimum possible file size. Evidently, Sharmila
>is using a MAXALIGN=4 machine and has a few dead rows in there.
Thanks for the detailed explanation Tom. You are rigt...The number of pages for
this tables as per pg_class is 189.
I have a few questions
1. How do you find the MAXALIGN of the machine? And what is that used for?
2. How does null columns account for this space. For example, if I have the
same table (10 cols --5 varchars and 5 numerics) in which 8 cols are null and 2
cols(1 varchar and 1 num) are not null, how is the storage space affected for
this case?
Thanks again
sharmila
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