1 files changed, 44 insertions, 0 deletions
diff --git a/src/include/access/rmgrdesc_utils.h b/src/include/access/rmgrdesc_utils.h
index 00e61464906..e09b1126740 100644
--- a/src/include/access/rmgrdesc_utils.h
+++ b/src/include/access/rmgrdesc_utils.h
@@ -12,6 +12,50 @@
 #ifndef RMGRDESC_UTILS_H_
 #define RMGRDESC_UTILS_H_
 
+/*
+ * Guidelines for rmgrdesc routine authors:
+ *
+ * The goal of these guidelines is to avoid gratuitous inconsistencies across
+ * each rmgr, and to allow users to parse desc output strings without too much
+ * difficulty.  This is not an API specification or an interchange format.
+ * (Only heapam and nbtree desc routines follow these guidelines at present,
+ * in any case.)
+ *
+ * Record descriptions are similar to JSON style key/value objects.  However,
+ * there is no explicit "string" type/string escaping.  Top-level { } brackets
+ * should be omitted.  For example:
+ *
+ * snapshotConflictHorizon: 0, flags: 0x03
+ *
+ * Record descriptions may contain variable-length arrays.  For example:
+ *
+ * nunused: 5, unused: [1, 2, 3, 4, 5]
+ *
+ * Nested objects are supported via { } brackets.  They generally appear
+ * inside variable-length arrays.  For example:
+ *
+ * ndeleted: 0, nupdated: 1, deleted: [], updated: [{ off: 45, nptids: 1, ptids: [0] }]
+ *
+ * Try to output things in an order that faithfully represents the order of
+ * fields from the underlying physical WAL record struct.  Key names should be
+ * unique (at the same nesting level) to make parsing easy.  It's a good idea
+ * if the number of items in the array appears before the array.
+ *
+ * It's okay for individual WAL record types to invent their own conventions.
+ * For example, Heap2's PRUNE record descriptions use a custom array format
+ * for the record's "redirected" field:
+ *
+ * ... redirected: [1->4, 5->9], dead: [10, 11], unused: [3, 7, 8]
+ *
+ * Arguably the desc routine should be using object notation for this instead.
+ * However, there is value in using a custom format when it conveys useful
+ * information about the underlying physical data structures.
+ *
+ * This ad-hoc format has the advantage of being close to the format used for
+ * the "dead" and "unused" arrays (which follow the standard desc convention
+ * for page offset number arrays).  It suggests that the "redirected" elements
+ * shown are just pairs of page offset numbers (which is how it really works).
+ */
 extern void array_desc(StringInfo buf, void *array, size_t elem_size, int count,
 					   void (*elem_desc) (StringInfo buf, void *elem, void *data),
 					   void *data);