That question doesn't really belong to the the novice list, does it? ;-)
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> -----Original Message-----
> From: pgsql-novice-owner@postgresql.org
> [mailto:pgsql-novice-owner@postgresql.org]On Behalf Of Manfred Koizar
> Sent: Thursday, May 02, 2002 11:23 AM
> To: pgsql-novice@postgresql.org
> Subject: [NOVICE] Trying to reduce per tuple overhead
>
>
> Hi,
>
> in htup.h MinHeapTupleBitmapSize is defined to be 32, i.e. the bitmap
> uses at least so many bits, if the tuple has at least one null
> attribute. The bitmap starts at offset 31 in the tuple header. The
> macro BITMAPLEN calculates, for a given number of attributes NATTS,
> the length of the bitmap in bytes. BITMAPLEN is the smallest number n
> divisible by 4, so that 8*n >= NATTS.
>
> The size of the tuple header is rounded up to a multiple of 4 (on a
> typical(?) architecture) by MAXALIGN(...). So we get:
>
> NATTS BITMAPLEN THSIZE
> 8 4 36
> 16 4 36
> 33 8 40
>
> I don't quite understand the definition of BITMAPLEN:
>
> #define BITMAPLEN(NATTS) \
> ((((((int)(NATTS) - 1) >> 3) + 4 - (MinHeapTupleBitmapSize >> 3)) \
> & ~03) + (MinHeapTupleBitmapSize >> 3))
>
> AFAICS only for MinHeapTupleBitmapSize == 32 we get a meaningful
> result, namely a multiple of MinHeapTupleBitmapSize, converted from a
> number of bits to a number of bytes. If this is true, we dont't need
> the "+ 4 - (MinHeapTupleBitmapSize >> 3)" and the definition could be
> simplified to
> (((((int)(NATTS) - 1) >> 3) & ~03) + 4)
>
> Some examples, writing MBMB for (MinHeapTupleBitmapSize >> 3):
>
> MBMB = 4:
> ((((NATTS - 1) >> 3) + 4 - MBMB) & ~03) + MBMB
> 32 31 3 7 3 0 4
> 33 32 4 8 4 4 8
> 64 63 7 11 7 4 8
> 65 64 8 12 8 8 12
>
> MBMB = 1:
> ((((NATTS - 1) >> 3) + 4 - MBMB) & ~03) + MBMB
> 8 7 0 4 3 0 1
> 9 8 1 5 4 4 5
> 32 31 3 7 6 4 5
> 33 32 4 8 7 4 5
> 56 55 6 10 9 8 9
> 64 63 7 11 10 8 9
> 65 64 8 12 11 8 9
>
> MBMB = 8:
> ((((NATTS - 1) >> 3) + 4 - MBMB) & ~03) + MBMB
> 8 7 0 4 -4 -4 4
> 9 8 1 5 -3 -4 4
> 32 31 3 7 -1 -4 4
> 33 32 4 8 0 0 8
> 56 55 6 10 2 0 8
> 64 63 7 11 3 0 8
> 65 64 8 12 4 4 12
>
> Proposal 1:
> #define BitMapBytes 4 // or any other power of 2
> #define MinHeapTupleBitmapSize (BitMapBytes * 8)
> #define BITMAPLEN(NATTS) \
> (((((int)(NATTS) - 1) >> 3) & ~(BitMapBytes - 1)) + BitMapBytes)
>
> Proposal 2: Let BITMAPLEN calculate the minimum number of bytes
> necessary to have one bit for every attribute.
>
> #define BitMapBytes 1
>
> old old new new
> NATTS BITMAPLEN THSIZE BITMAPLEN THSIZE
> 8 4 36 1 32
> 16 4 36 2 36
> 33 8 40 5 36
>
> This looks so simple. Is there something wrong with it?
>
> Servus
> Manfred
>
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