Remove entry tree root conflict checking from GIN predicate locking

According to README we acquire predicate locks on entry tree leafs and posting
tree roots.  However, when ginFindLeafPage() is going to lock leaf in exclusive
mode, then it checks root for conflicts regardless whether it's a entry or
posting tree.  Assuming that we never place predicate lock on entry tree root
(excluding corner case when root is leaf), this check is redundant.  This
commit removes this check.  Now, root conflict checking is controlled by
separate argument of ginFindLeafPage().

Discussion: https://postgr.es/m/CAPpHfdv7rrDyy%3DMgsaK-L9kk0AH7az0B-mdC3w3p0FSb9uoyEg%40mail.gmail.com
Author: Alexander Korotkov
Backpatch-through: 11

5 files changed, 13 insertions, 8 deletions

diff --git a/src/backend/access/gin/ginbtree.c b/src/backend/access/gin/ginbtree.c
index 030d0f44183..78213dcb4ef 100644
--- a/src/backend/access/gin/ginbtree.c
+++ b/src/backend/access/gin/ginbtree.c
@@ -72,9 +72,13 @@ ginTraverseLock(Buffer buffer, bool searchMode)
  * If 'searchmode' is false, on return stack->buffer is exclusively locked,
  * and the stack represents the full path to the root. Otherwise stack->buffer
  * is share-locked, and stack->parent is NULL.
+ *
+ * If 'rootConflictCheck' is true, tree root is checked for serialization
+ * conflict.
  */
 GinBtreeStack *
-ginFindLeafPage(GinBtree btree, bool searchMode, Snapshot snapshot)
+ginFindLeafPage(GinBtree btree, bool searchMode,
+				bool rootConflictCheck, Snapshot snapshot)
 {
 	GinBtreeStack *stack;
 
@@ -84,7 +88,7 @@ ginFindLeafPage(GinBtree btree, bool searchMode, Snapshot snapshot)
 	stack->parent = NULL;
 	stack->predictNumber = 1;
 
-	if (!searchMode)
+	if (rootConflictCheck)
 		CheckForSerializableConflictIn(btree->index, NULL, stack->buffer);
 
 	for (;;)
diff --git a/src/backend/access/gin/gindatapage.c b/src/backend/access/gin/gindatapage.c
index 9f20513811e..54d68909125 100644
--- a/src/backend/access/gin/gindatapage.c
+++ b/src/backend/access/gin/gindatapage.c
@@ -1913,7 +1913,7 @@ ginInsertItemPointers(Relation index, BlockNumber rootBlkno,
 	{
 		/* search for the leaf page where the first item should go to */
 		btree.itemptr = insertdata.items[insertdata.curitem];
-		stack = ginFindLeafPage(&btree, false, NULL);
+		stack = ginFindLeafPage(&btree, false, true, NULL);
 
 		ginInsertValue(&btree, stack, &insertdata, buildStats);
 	}
@@ -1932,7 +1932,7 @@ ginScanBeginPostingTree(GinBtree btree, Relation index, BlockNumber rootBlkno,
 
 	btree->fullScan = true;
 
-	stack = ginFindLeafPage(btree, true, snapshot);
+	stack = ginFindLeafPage(btree, true, false, snapshot);
 
 	return stack;
 }
diff --git a/src/backend/access/gin/ginget.c b/src/backend/access/gin/ginget.c
index 8466d947eab..6bfe18c359f 100644
--- a/src/backend/access/gin/ginget.c
+++ b/src/backend/access/gin/ginget.c
@@ -338,7 +338,7 @@ restartScanEntry:
 	ginPrepareEntryScan(&btreeEntry, entry->attnum,
 						entry->queryKey, entry->queryCategory,
 						ginstate);
-	stackEntry = ginFindLeafPage(&btreeEntry, true, snapshot);
+	stackEntry = ginFindLeafPage(&btreeEntry, true, false, snapshot);
 	page = BufferGetPage(stackEntry->buffer);
 
 	/* ginFindLeafPage() will have already checked snapshot age. */
@@ -679,7 +679,7 @@ entryLoadMoreItems(GinState *ginstate, GinScanEntry entry,
 						   OffsetNumberNext(GinItemPointerGetOffsetNumber(&advancePast)));
 		}
 		entry->btree.fullScan = false;
-		stack = ginFindLeafPage(&entry->btree, true, snapshot);
+		stack = ginFindLeafPage(&entry->btree, true, false, snapshot);
 
 		/* we don't need the stack, just the buffer. */
 		entry->buffer = stack->buffer;
diff --git a/src/backend/access/gin/gininsert.c b/src/backend/access/gin/gininsert.c
index 5281eb68238..2781d3fdd83 100644
--- a/src/backend/access/gin/gininsert.c
+++ b/src/backend/access/gin/gininsert.c
@@ -195,7 +195,7 @@ ginEntryInsert(GinState *ginstate,
 
 	ginPrepareEntryScan(&btree, attnum, key, category, ginstate);
 
-	stack = ginFindLeafPage(&btree, false, NULL);
+	stack = ginFindLeafPage(&btree, false, false, NULL);
 	page = BufferGetPage(stack->buffer);
 
 	if (btree.findItem(&btree, stack))
diff --git a/src/include/access/gin_private.h b/src/include/access/gin_private.h
index 81bf8734ce3..bca82463199 100644
--- a/src/include/access/gin_private.h
+++ b/src/include/access/gin_private.h
@@ -195,7 +195,8 @@ typedef struct
  * PostingItem
  */
 
-extern GinBtreeStack *ginFindLeafPage(GinBtree btree, bool searchMode, Snapshot snapshot);
+extern GinBtreeStack *ginFindLeafPage(GinBtree btree, bool searchMode,
+				bool rootConflictCheck, Snapshot snapshot);
 extern Buffer ginStepRight(Buffer buffer, Relation index, int lockmode);
 extern void freeGinBtreeStack(GinBtreeStack *stack);
 extern void ginInsertValue(GinBtree btree, GinBtreeStack *stack,