细说vacuum的truncate操作

/*
 * lazy_truncate_heap - try to truncate off any empty pages at the end
 */
static void
lazy_truncate_heap(LVRelState *vacrel)
{
	BlockNumber old_rel_pages = vacrel->rel_pages;
	BlockNumber new_rel_pages;
	bool		lock_waiter_detected;
	int			lock_retry;

	/* Report that we are now truncating */
	pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
								 PROGRESS_VACUUM_PHASE_TRUNCATE);

	/*
	 * Loop until no more truncating can be done.
	 */
	do
	{
		PGRUsage	ru0;

		pg_rusage_init(&ru0);

		/*
		 * We need full exclusive lock on the relation in order to do
		 * truncation. If we can't get it, give up rather than waiting --- we
		 * don't want to block other backends, and we don't want to deadlock
		 * (which is quite possible considering we already hold a lower-grade
		 * lock).
		 */
		lock_waiter_detected = false;
		lock_retry = 0;
		while (true)
		{
			if (ConditionalLockRelation(vacrel->rel, AccessExclusiveLock))
				break;

			/*
			 * Check for interrupts while trying to (re-)acquire the exclusive
			 * lock.
			 */
			CHECK_FOR_INTERRUPTS();

			if (++lock_retry > (VACUUM_TRUNCATE_LOCK_TIMEOUT /
								VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL))
			{
				/*
				 * We failed to establish the lock in the specified number of
				 * retries. This means we give up truncating.
				 */
				ereport(elevel,
						(errmsg("\"%s\": stopping truncate due to conflicting lock request",
								vacrel->relname)));
				return;
			}

			pg_usleep(VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL * 1000L);
		}

		/*
		 * Now that we have exclusive lock, look to see if the rel has grown
		 * whilst we were vacuuming with non-exclusive lock.  If so, give up;
		 * the newly added pages presumably contain non-deletable tuples.
		 */
		new_rel_pages = RelationGetNumberOfBlocks(vacrel->rel);
		if (new_rel_pages != old_rel_pages)
		{
			/*
			 * Note: we intentionally don't update vacrel->rel_pages with the
			 * new rel size here.  If we did, it would amount to assuming that
			 * the new pages are empty, which is unlikely. Leaving the numbers
			 * alone amounts to assuming that the new pages have the same
			 * tuple density as existing ones, which is less unlikely.
			 */
			UnlockRelation(vacrel->rel, AccessExclusiveLock);
			return;
		}

		/*
		 * Scan backwards from the end to verify that the end pages actually
		 * contain no tuples.  This is *necessary*, not optional, because
		 * other backends could have added tuples to these pages whilst we
		 * were vacuuming.
		 */
		new_rel_pages = count_nondeletable_pages(vacrel, &lock_waiter_detected);
		vacrel->blkno = new_rel_pages;

		if (new_rel_pages >= old_rel_pages)
		{
			/* can't do anything after all */
			UnlockRelation(vacrel->rel, AccessExclusiveLock);
			return;
		}

		/*
		 * Okay to truncate.
		 */
		RelationTruncate(vacrel->rel, new_rel_pages);

		/*
		 * We can release the exclusive lock as soon as we have truncated.
		 * Other backends can't safely access the relation until they have
		 * processed the smgr invalidation that smgrtruncate sent out ... but
		 * that should happen as part of standard invalidation processing once
		 * they acquire lock on the relation.
		 */
		UnlockRelation(vacrel->rel, AccessExclusiveLock);

		/*
		 * Update statistics.  Here, it *is* correct to adjust rel_pages
		 * without also touching reltuples, since the tuple count wasn't
		 * changed by the truncation.
		 */
		vacrel->pages_removed += old_rel_pages - new_rel_pages;
		vacrel->rel_pages = new_rel_pages;

		ereport(elevel,
				(errmsg("table \"%s\": truncated %u to %u pages",
						vacrel->relname,
						old_rel_pages, new_rel_pages),
				 errdetail_internal("%s",
									pg_rusage_show(&ru0))));
		old_rel_pages = new_rel_pages;
	} while (new_rel_pages > vacrel->nonempty_pages && lock_waiter_detected);
}