transaction wraparound

Hi.

I apologize for my English.

I have a question about transaction wraparound. As far as I know, transaction numbers are 32 bit, which greatly limits their number. In the tuple header there are now t_xmin and t_xmax fields - both 32 bits each. So why not replace them with a single field with a transaction number of 64 bits length - this is the transaction number that created the version ( this is exactly what the MS SQL server does)? In this case, you do not have to lengthen header tuple. Of course, this can result in increased I/O, so to compensate for it, you can turn off buffered read (O_DIRECT) while reading.