The first is heapgettup() and the other is heap_fetch().
I'm guessing that one (heapgettup()) is used for sequential scans and the other (heap_fetch()) is used to retreive a tuple based on an index entry. Is that right? (or close to right - I would guess that heap_fetch() might also be called for other purposes).
Each of these functions calls HeapTupleSatisfies().
My (novice) interpretation of this code is that, if HeapTupleSatisfies() succeeds (that is, it sets the 'valid' parameter to TRUE), the given tuple is visible to the current transaction. If HeapTupleSatisfies() fails (sets 'valid' to FALSE), the tuple is *not* visible to the current transaction.
HeapTupleSatisfies() will return FALSE for a tuple that has been deleted but not yet VACUUMed away, or for a tuple that has been modified (or added) by another transaction that has not yet committed, or for a tuple that has been committed by another transaction but it was committed since our current command started scanning. (There are probably some other rules that I can glean from the documentation, but if I got any of those wrong, can someone correct me?)
If HeapTupleSatisfies() returns FALSE, that means that we have read a page from disk that contains at least one invisible tuple. If I read a page that contains only invisible tuples, that was a wasted I/O (at least from my transaction's perspective).
Did I get (most or any of) this right?
TIA - Murphy
_________________________________________________________________
Concerned that messages may bounce because your Hotmail account has exceeded its 2MB storage limit? Get Hotmail Extra Storage! http://join.msn.com/?PAGE=features/es
---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings
