nbtree: Always set skipScan flag on rescan.

petergeoghegan · petergeoghegan · commit bee763aea13f · 2025-09-13T21:01:31.000-04:00
The TimescaleDB extension expects to be able to change an nbtree scan's keys across rescans. The issue arises in the extension's implementation of loose index scan. This is arguably a misuse of the index AM API, though apparently it worked until recently. It stopped working when the skipScan flag was added to BTScanOpaqueData by commit 8a51027, though. The flag wouldn't reliably track whether the scan (actually, the current rescan) has any skip arrays, leading to confusion in _bt_set_startikey. nbtree preprocessing will now defensively initialize the scan's skipScan flag in all cases, including the case where _bt_preprocess_array_keys returns early due to the (re)scan not using arrays. While nbtree isn't obligated to support this use case (at least not according to my reading of the index AM API), it still seems like a good idea to be consistent here, on general robustness grounds. Author: Peter Geoghegan <pg@bowt.ie> Reported-By: Natalya Aksman <natalya@timescale.com> Discussion: https://postgr.es/m/CAJumhcirfMojbk20+W0YimbNDkwdECvJprQGQ-XqK--ph09nQw@mail.gmail.com Backpatch-through: 18
diff --git a/src/backend/access/nbtree/nbtpreprocesskeys.c b/src/backend/access/nbtree/nbtpreprocesskeys.c
@@ -1852,6 +1852,7 @@ _bt_preprocess_array_keys(IndexScanDesc scan, int *new_numberOfKeys)
 	 * (also checks if we should add extra skip arrays based on input keys)
 	 */
 	numArrayKeys = _bt_num_array_keys(scan, skip_eq_ops, &numSkipArrayKeys);
+	so->skipScan = (numSkipArrayKeys > 0);
 
 	/* Quit if nothing to do. */
 	if (numArrayKeys == 0)
@@ -1881,7 +1882,6 @@ _bt_preprocess_array_keys(IndexScanDesc scan, int *new_numberOfKeys)
 	arrayKeyData = (ScanKey) palloc(numArrayKeyData * sizeof(ScanKeyData));
 
 	/* Allocate space for per-array data in the workspace context */
-	so->skipScan = (numSkipArrayKeys > 0);
 	so->arrayKeys = (BTArrayKeyInfo *) palloc(numArrayKeys * sizeof(BTArrayKeyInfo));
 
 	/* Allocate space for ORDER procs used to help _bt_checkkeys */
