amcheck: Optimize speed of checking for unique constraint violation

Currently, when amcheck validates a unique constraint, it visits the heap for
each index tuple.  This commit implements skipping keys, which have only one
non-dedeuplicated index tuple (quite common case for unique indexes). That
gives substantial economy on index checking time.

Reported-by: Noah Misch
Discussion: https://postgr.es/m/20240325020323.fd.nmisch%40google.com
Author: Alexander Korotkov, Pavel Borisov

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 34990c5cea3f3605bc54966dc94bef2ea632f864..7cfb136763f6493b62641e2b087e986e53e604d4 100644 (file)
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1433,6 +1433,13 @@ bt_target_page_check(BtreeCheckState *state)
        bool        lowersizelimit;
        ItemPointer scantid;
 
+       /*
+        * True if we already called bt_entry_unique_check() for the current
+        * item.  This helps to avoid visiting the heap for keys, which are
+        * anyway presented only once and can't comprise a unique violation.
+        */
+       bool        unique_checked = false;
+
        CHECK_FOR_INTERRUPTS();
 
        itemid = PageGetItemIdCareful(state, state->targetblock,
@@ -1775,12 +1782,18 @@ bt_target_page_check(BtreeCheckState *state)
 
        /*
         * If the index is unique verify entries uniqueness by checking the
-        * heap tuples visibility.
+        * heap tuples visibility.  Immediately check posting tuples and
+        * tuples with repeated keys.  Postpone check for keys, which have the
+        * first appearance.
         */
        if (state->checkunique && state->indexinfo->ii_Unique &&
-           P_ISLEAF(topaque) && !skey->anynullkeys)
+           P_ISLEAF(topaque) && !skey->anynullkeys &&
+           (BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis.tid)))
+       {
            bt_entry_unique_check(state, itup, state->targetblock, offset,
                                  &lVis);
+           unique_checked = true;
+       }
 
        if (state->checkunique && state->indexinfo->ii_Unique &&
            P_ISLEAF(topaque) && OffsetNumberNext(offset) <= max)
@@ -1799,6 +1812,9 @@ bt_target_page_check(BtreeCheckState *state)
             * data (whole index tuple or last posting in index tuple). Key
             * containing null value does not violate unique constraint and
             * treated as different to any other key.
+            *
+            * If the next key is the same as the previous one, do the
+            * bt_entry_unique_check() call if it was postponed.
             */
            if (_bt_compare(state->rel, skey, state->target,
                            OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
@@ -1808,6 +1824,11 @@ bt_target_page_check(BtreeCheckState *state)
                lVis.postingIndex = -1;
                lVis.tid = NULL;
            }
+           else if (!unique_checked)
+           {
+               bt_entry_unique_check(state, itup, state->targetblock, offset,
+                                     &lVis);
+           }
            skey->scantid = scantid;    /* Restore saved scan key state */
        }
 
@@ -1890,10 +1911,19 @@ bt_target_page_check(BtreeCheckState *state)
                rightkey->scantid = NULL;
 
                /* The first key on the next page is the same */
-               if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+               if (_bt_compare(state->rel, rightkey, state->target, max) == 0 &&
+                   !rightkey->anynullkeys)
                {
                    Page        rightpage;
 
+                   /*
+                    * Do the bt_entry_unique_check() call if it was
+                    * postponed.
+                    */
+                   if (!unique_checked)
+                       bt_entry_unique_check(state, itup, state->targetblock,
+                                             offset, &lVis);
+
                    elog(DEBUG2, "cross page equal keys");
                    rightpage = palloc_btree_page(state,
                                                  rightblock_number);