From 1c15aac53f3421fd38ae145118d3204f055ba955 Mon Sep 17 00:00:00 2001
From: Kevin Grittner
Date: Tue, 19 Jul 2016 16:25:53 -0500
Subject: Add comment & docs about no vacuum truncation with sto.

Omission noted by Andres Freund.
---
 src/backend/commands/vacuumlazy.c | 9 +++++++++
 1 file changed, 9 insertions(+)

(limited to 'src')

diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index 4075f4d1c92..231e92d8e46 100644
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -1663,6 +1663,15 @@ lazy_cleanup_index(Relation indrel,
  * Don't even think about it unless we have a shot at releasing a goodly
  * number of pages.  Otherwise, the time taken isn't worth it.
  *
+ * Also don't attempt it if we are doing early pruning/vacuuming, because a
+ * scan which cannot find a truncated heap page cannot determine that the
+ * snapshot is too old to read that page.  We might be able to get away with
+ * truncating all except one of the pages, setting its LSN to (at least) the
+ * maximum of the truncated range if we also treated an index leaf tuple
+ * pointing to a missing heap page as something to trigger the "snapshot too
+ * old" error, but that seems fragile and seems like it deserves its own patch
+ * if we consider it.
+ *
  * This is split out so that we can test whether truncation is going to be
  * called for before we actually do it.  If you change the logic here, be
  * careful to depend only on fields that lazy_scan_heap updates on-the-fly.
-- 
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