Paul Tillotson <[EMAIL PROTECTED]> writes: > For some time, I have wondered: what does postgres use "t_ctid chains" > for? It seems like it is useful to find the "newer" version of a > tuple. However, wouldn't that eventually get found anyway? A > sequential scan scans the whole table, and so it will find the new > tuple. Since indexes contain all tuples, so will an index scan.
The problem is not that the table reader wouldn't "find" the tuple. The problem is that he'd disregard it as too new for his snapshot. The essential point of the EvalPlanQual mechanism is to identify tuples that we should consider visible for modification even though the MVCC rules say no. Basically, the normal search mechanisms will find a prior state of the row (whichever state was committed when we took our snapshot) and then we have to "chain up" to the latest state by following the t_ctid links. There's some discussion of this in the manual under http://developer.postgresql.org/docs/postgres/transaction-iso.html#XACT-READ-COMMITTED regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 4: Have you searched our list archives? http://archives.postgresql.org
