On 4.4.2011 21:30, Norman Dunbar wrote: > Hi Boris, > > On 04/04/11 19:08, dbori...@hotmail.com wrote: > >> I have (maybe silly) question about NULL mask. > There are no silly questions! > > <SNIP> > >> First record NULL mask (Dpg_offset was 0x0FE0): >> 01 FC FD 00 (FC 00 00 00) - OK, >> Second record NULL mask: >> 01 FE FD 00 (FE 00 00 00) - OK >> Third record NULL mask: >> 01 FF E9 00 -??? - this part I do not understand. >> >> If I understand correctly, there is E9 (23) bytes with zeros in NULL >> mask? NULL mask should occupy 4 or 8 or 12 etc bytes. What am I missing? > Nothing at all. There will be 23 zero bytes in this part of the page. > > However, they are simply padding in the case of the example record > contents you supplied - two NULL fields. The data in entire record is > effectively covered by the bits in the NULL mask. > > The zero bytes are most likely the initialised contents of the working > buffer where the data was assembled before being written to the page. > > If you were to update the data in one or both of the columns, you would > be able to examine the hex dump again and see that the NULL mask now > shows not null, and the existing zeros will hold the compressed format > of the new data. > > The NULL mask is a minimum of 4 bytes to allow for up to 32 fields. If > there is 33 fields, then the NULL mask will consist of another 4 bytes, > of which, only 1 single bit will be used. And so on, adding 4 bytes > every time there is more than a multiple of 32 fields in a record. > > > >> Thanks in advance and sorry for my English > Your English is fine, trust me. I speak *only* English I'm ashamed to say. > > HTH > > Cheers, > Norman. > Thank you, Boris.
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