On Sat, Sep 23, 2023 at 11:47 AM Peter Geoghegan <p...@bowt.ie> wrote: > The fix for this should be fairly straightforward. We must teach > _bt_restore_array_keys() to distinguish "past the end of the array" > from "after the start of the array", so that doesn't spuriously skip a > required call to _bt_preprocess_keys() . I already see that the > problem goes away once _bt_restore_array_keys() is made to call > _bt_preprocess_keys() unconditionally, so I'm already fairly confident > that this will work.
Attached draft patch shows how this could work. _bt_restore_array_keys() has comments that seem to suppose that calling _bt_preprocess_keys is fairly expensive, and something that's well worth avoiding. But...is it, really? I wonder if we'd be better off just biting the bullet and always calling _bt_preprocess_keys here. Could it really be such a big cost, compared to pinning and locking the page in the btrestpos() path? The current draft SAOP patch calls _bt_preprocess_keys() with a buffer lock held. This is very likely unsafe, so I'll need to come up with a principled approach (e.g. a stripped down, specialized version of _bt_preprocess_keys that's safe to call while holding a lock seems doable). I've been able to put that off for a few months now because it just doesn't impact my microbenchmarks to any notable degree (and not just in those cases where we can use the "numberOfKeys == 1" fast path at the start of _bt_preprocess_keys). This at least suggests that the cost of always calling _bt_preprocess_keys in _bt_restore_array_keys might be acceptable. -- Peter Geoghegan
From 781fd293aa7556f5ab029d2d5c2dfc829a4e8efd Mon Sep 17 00:00:00 2001 From: Peter Geoghegan <pg@bowt.ie> Date: Sat, 23 Sep 2023 15:00:59 -0700 Subject: [PATCH v1] Fix btmarkpos/btrestrpos array keys edge case. Oversight in bug fix commit 70bc5833, which taught nbtree to handle array keys during mark/restore processing, but missed this subtlety. Author: Peter Geoghegan <pg@bowt.ie> Discussion: CAH2-WzkgP3DDRJxw6DgjCxo-cu-DKrvjEv_ArkP2ctBJatDCYg@mail.gmail.com">https://postgr.es/m/CAH2-WzkgP3DDRJxw6DgjCxo-cu-DKrvjEv_ArkP2ctBJatDCYg@mail.gmail.com Backpatch: 11- (all supported branches). --- src/include/access/nbtree.h | 2 ++ src/backend/access/nbtree/nbtree.c | 1 + src/backend/access/nbtree/nbtutils.c | 18 ++++++++++++++---- 3 files changed, 17 insertions(+), 4 deletions(-) diff --git a/src/include/access/nbtree.h b/src/include/access/nbtree.h index f5c66964c..b38279e7b 100644 --- a/src/include/access/nbtree.h +++ b/src/include/access/nbtree.h @@ -1043,6 +1043,8 @@ typedef struct BTScanOpaqueData /* workspace for SK_SEARCHARRAY support */ ScanKey arrayKeyData; /* modified copy of scan->keyData */ + bool arraysStarted; /* Started array keys, but have yet to + * "reach past the end" of all arrays? */ int numArrayKeys; /* number of equality-type array keys (-1 if * there are any unsatisfiable array keys) */ int arrayKeyCount; /* count indicating number of array scan keys diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c index 62bc9917f..6c5b5c69c 100644 --- a/src/backend/access/nbtree/nbtree.c +++ b/src/backend/access/nbtree/nbtree.c @@ -364,6 +364,7 @@ btbeginscan(Relation rel, int nkeys, int norderbys) so->keyData = NULL; so->arrayKeyData = NULL; /* assume no array keys for now */ + so->arraysStarted = false; so->numArrayKeys = 0; so->arrayKeys = NULL; so->arrayContext = NULL; diff --git a/src/backend/access/nbtree/nbtutils.c b/src/backend/access/nbtree/nbtutils.c index 7da499c4d..17e597f11 100644 --- a/src/backend/access/nbtree/nbtutils.c +++ b/src/backend/access/nbtree/nbtutils.c @@ -539,6 +539,8 @@ _bt_start_array_keys(IndexScanDesc scan, ScanDirection dir) curArrayKey->cur_elem = 0; skey->sk_argument = curArrayKey->elem_values[curArrayKey->cur_elem]; } + + so->arraysStarted = true; } /* @@ -598,6 +600,14 @@ _bt_advance_array_keys(IndexScanDesc scan, ScanDirection dir) if (scan->parallel_scan != NULL) _bt_parallel_advance_array_keys(scan); + /* + * When no new array keys were found, the scan is "past the end" of the + * array keys. _bt_start_array_keys can still "restart" the array keys if + * a rescan is required. + */ + if (!found) + so->arraysStarted = false; + return found; } @@ -648,11 +658,11 @@ _bt_restore_array_keys(IndexScanDesc scan) } /* - * If we changed any keys, we must redo _bt_preprocess_keys. That might - * sound like overkill, but in cases with multiple keys per index column - * it seems necessary to do the full set of pushups. + * If we changed any keys, or if we're not sure that so->keyData[] + * contains the array keys indicated as current within so->arrayKeys[], + * then we must redo _bt_preprocess_keys */ - if (changed) + if (changed || !so->arraysStarted) { _bt_preprocess_keys(scan); /* The mark should have been set on a consistent set of keys... */ -- 2.40.1
