On Wed, Apr 6, 2016 at 6:33 AM, Albe Laurenz <laurenz.albe@xxxxxxxxxx> wrote: > Rakesh Kumar wrote: > Every row has two system columns associated with it: xmin and xmax > > xmin is the transaction ID that created the row, while xmax is > the transaction ID that removed the row. > > So when an update takes place, xmax of the original row and xmin > of the new row are set to the transaction ID of the current transaction. > > Furthermore, the commit log (CLOG) logs for each transaction whether > it was committed or rolled back. > > Now when a backend examines a row, it first checks if the row is > visible, i.e. xmin must be less or equal to the current transaction ID > and xmax must be 0 or greater than the current transaction ID > or belong to a transaction that was rolled back. > > To save CLOG lookups, the first reader who consults the CLOG will > save the result in so-called "hint bits" on the row itself. I am assuming the same must be true for the indexes also. Does PG look up primary key by examining the rows like you described above. -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
