Add explanatory comment to prune_freeze_setup()

heap_page_prune_and_freeze() fills in PruneState->deadoffsets, the array
of OffsetNumbers of dead tuples. It is returned to the caller in the
PruneFreezeResult. To avoid having two copies of the array, the
PruneState saves only a pointer to the array. This was a bit unusual and
confusing, so add a clarifying comment.

Author: Melanie Plageman <[email protected]>
Suggested-by: Chao Li <[email protected]>
Discussion: https://postgr.es/m/CAEoWx2=jiD1nqch4JQN+odAxZSD7mRvdoHUGJYN2r6tQG_66yQ@mail.gmail.com

diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index f99d92725b8841cb59549ceaabe6b0d310f3330a..07aa08cfe14d6ef4dd730fae1b2f4511d3a5f902 100644 (file)
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -385,6 +385,13 @@ prune_freeze_setup(PruneFreezeParams *params,
    prstate->recently_dead_tuples = 0;
    prstate->hastup = false;
    prstate->lpdead_items = 0;
+
+   /*
+    * deadoffsets are filled in during pruning but are only used to populate
+    * PruneFreezeResult->deadoffsets. To avoid needing two copies of the
+    * array, just save a pointer to the result offsets array in the
+    * PruneState.
+    */
    prstate->deadoffsets = presult->deadoffsets;
    prstate->frz_conflict_horizon = InvalidTransactionId;