Re: Hot Standby (commit fest version - v5)

On Sat, Nov 1, 2008 at 10:02 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:

> Hot Standby patch, including all major planned features.
>
>
>
I wonder if we should refactor lazy_scan_heap() so that *all* the real work
of collecting information about dead tuples happens only in
heap_page_prune(). Frankly, there is only a rare chance that a tuple may
become DEAD after the pruning happened on the page. We can ignore such
tuples; they will be vacuumed/pruned in the next cycle.

This would save us a second check of HeapTupleSatisfiesVacuum on the tuples
which are just now checked in heap_page_prune(). In addition, the following
additional WAL records are then not necessary because heap_page_prune() must
have already logged the latestRemovedXid.

+ /*
+ * For Hot Standby we need to know the highest transaction id that will
+ * be removed by any change. VACUUM proceeds in a number of passes so
+ * we need to consider how each pass operates. The first pass runs
+ * heap_page_prune(), which can issue XLOG_HEAP2_CLEAN records as it
+ * progresses - these will have a latestRemovedXid on each record.
+ * In many cases this removes all of the tuples to be removed.
+ * Then we look at tuples to be removed, but do not actually remove them
+ * until phase three. However, index records for those rows are removed
+ * in phase two and index blocks do not have MVCC information attached.
+ * So before we can allow removal of *any* index tuples we need to issue
+ * a WAL record indicating what the latestRemovedXid will be at the end
+ * of phase three. This then allows Hot Standby queries to block at the
+ * correct place, i.e. before phase two, rather than during phase three
+ * as we issue more XLOG_HEAP2_CLEAN records. If we need to run multiple
+ * phase two/three because of memory constraints we need to issue multiple
+ * log records also.
+ */
+ static void
+ vacuum_log_cleanup_info(Relation rel, LVRelStats *vacrelstats)
+ {
+ /*
+ * No need to log changes for temp tables, they do not contain
+ * data visible on the standby server.
+ */
+ if (rel->rd_istemp)
+ return;
+
+ (void) log_heap_cleanup_info(rel->rd_node,
vacrelstats->latestRemovedXid);
+ }

Thanks,
Pavan

--
Pavan Deolasee
EnterpriseDB http://www.enterprisedb.com