Martín On Tue, Nov 20th, 2018 at 6:0 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> > > > =?UTF-8?q?Mart=C3=ADn_Fern=C3=A1ndez?= < fmarti...@gmail.com > writes: > > First thing that generated a lot of noise in my head was the following, > if pg assigns contiguous numeric values for the txid, how does pg deal > with fragmentation issues ? Then I later found that the txid space is > actually circular and not linearly as I originally thought it was. This > exposed me to the fact that the txid is actually exported as a 64bit value > where the last 32bits are an epoch. My understanding is that the epoch is > the component that allows the circularity of the data structure. I then > started analyzing how pg decides if a given tuple is eligible for > freezing. I found out that pg will compare a cutoff_tx (I assume this is > the last committed tx) with the xmin value of the given tuple, if xmin > precedes the cutoff_tx the tuple is eligible (I’m ignoring > HEAP_XMAX_IS_MULTI and HEAP_MOVED cases). Now, the xmin of a tuple is an > 32 bit integer, so, how is the epoch part of an exported txid considered > here ? What if we had a database really old where a txid with integer > value 10 is greater than a txid of value 1000 ? > > Actually, XID epoch is an artifact that's bolted on for possible use by > replication or what have you. So far as the core database is concerned, > XIDs are 32 bits in a circular space, and the way that we deal with your > question is we don't let the case arise. Every old tuple must be marked > "frozen" before its XID gets to be 2 billion XIDs old; after that, we > don't particularly care just how old it is. The whole "wraparound" > business just exists to make sure that happens in time. > > If the stored XIDs were 64 bits wide, we'd not have to bother with all > of this mess ... but adding another 64 bits to tuple headers would be > a painful space cost, not to mention the storage compatibility issues. > > regards, tom lane > > > > Tom, Thanks for the insight!! I got confused with the comment under "Transaction IDs and Snapshots"( https://www.postgresql.org/docs/current/functions-info.html ) "The internal transaction ID type (xid) is 32 bits wide and wraps around every 4 billion transactions. However, these functions export a 64-bit format that is extended with an "epoch" counter so it will not wrap around during the life of an installation. "
