Clarify nbtree posting list update desc issue.

Per complaint from Melanie Plageman.

Follow-up to commit 5d6728e5.

Reported-By: Melanie Plageman <[email protected]>
Discussion: https://postgr.es/m/20230411002315.oyaicmcqrq2hb3ek@liskov

diff --git a/src/backend/access/rmgrdesc/nbtdesc.c b/src/backend/access/rmgrdesc/nbtdesc.c
index 0db3274dc512328d08d304fd0c772077c62d2620..c50d5547de86462e50ead987f17870e390bc4f5c 100644 (file)
--- a/src/backend/access/rmgrdesc/nbtdesc.c
+++ b/src/backend/access/rmgrdesc/nbtdesc.c
@@ -223,6 +223,11 @@ delvacuum_desc(StringInfo buf, char *block_data,
        Assert(OffsetNumberIsValid(off));
        Assert(updates->ndeletedtids > 0);
 
+       /*
+        * "ptid" is the symbol name used when building each xl_btree_update's
+        * array of offsets into a posting list tuple's ItemPointerData array.
+        * xl_btree_update describes a subset of the existing TIDs to delete.
+        */
        appendStringInfo(buf, "{ off: %u, nptids: %u, ptids: [",
                         off, updates->ndeletedtids);
        for (int p = 0; p < updates->ndeletedtids; p++)