Manfred Koizar wrote: > Tom Lane wrote in another tread: > > PS: I did like your point about BITMAPLEN; I think that might be > > a free savings. I was waiting for you to bring it up on hackers > > before commenting though... > So here we go... > > 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))
Thanks for improving this. I had to look at this macro recently and it was quite confusing. -- Bruce Momjian | http://candle.pha.pa.us [EMAIL PROTECTED] | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026 ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org
