Subject: Re: CURRENT OF causes an error when IndexOnlyScan is used
To: Tom Lane <tgl@sss.pgh.pa.us>, Yugo Nagata <nagata@sraoss.co.jp>
Cc: pgsql-hackers@postgresql.org
References: <20180201013349.937dfc5f.nagata@sraoss.co.jp>
 <20180201014444.f78358df.nagata@sraoss.co.jp>
 <22958.1517451171@sss.pgh.pa.us>
From: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>
Message-ID: <97dc81d3-0ea9-3032-49d9-7983e35fb010@postgrespro.ru>
Date: Mon, 19 Feb 2018 17:36:55 +0300
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01.02.2018 05:12, Tom Lane:
> Yugo Nagata <nagata@sraoss.co.jp> writes:
>> I'm sorry the patch attached in the previous mail is broken and
>> not raises a compile error. I attached the fixed patch.
> This patch is almost certainly wrong: you can't assume that the scan-level
> state matches the tuple we are currently processing at top level.  Any
> sort of delaying action, for instance a sort or materialize node in
> between, would break it.
>
> We need to either fix this aspect:
>
>>> IndexOnlyScan returns a virtual tuple that doesn't have system
>>> column, so we can not get ctid in the same way of other plans.

I'd like to propose the patch that fixes the issue.
We already have a way to return heaptuple from IndexOnlyScan,
but it was not applied to b-tree for some reason.

Attached patch solves the reported bug.
Moreover, it will come in handy for "index with included attributes" 
feature [1],
where we can store long (and even TOASTed) attributes in indextuple.

[1] https://commitfest.postgresql.org/17/1350/

-- 
Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company


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diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
index 8158508..db8a55c 100644
--- a/src/backend/access/nbtree/nbtree.c
+++ b/src/backend/access/nbtree/nbtree.c
@@ -374,6 +374,8 @@ btbeginscan(Relation rel, int nkeys, int norderbys)
 	so->currTuples = so->markTuples = NULL;
 
 	scan->xs_itupdesc = RelationGetDescr(rel);
+	scan->xs_hitupdesc = NULL;
+	scan->xs_hitup = NULL;
 
 	scan->opaque = so;
 
diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 51dca64..dd3e8b2 100644
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -25,6 +25,7 @@
 #include "utils/tqual.h"
 
 
+static HeapTuple _bt_fetch_tuple(IndexScanDesc scandesc);
 static bool _bt_readpage(IndexScanDesc scan, ScanDirection dir,
 			 OffsetNumber offnum);
 static void _bt_saveitem(BTScanOpaque so, int itemIndex,
@@ -38,6 +39,31 @@ static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);
 static void _bt_drop_lock_and_maybe_pin(IndexScanDesc scan, BTScanPos sp);
 static inline void _bt_initialize_more_data(BTScanOpaque so, ScanDirection dir);
 
+/*
+ * Fetch all keys in tuple.
+ * Returns a new HeapTuple containing the originally-indexed data.
+ */
+static HeapTuple
+_bt_fetch_tuple(IndexScanDesc scandesc)
+{
+	Relation index = scandesc->indexRelation;
+	Datum		fetchatt[INDEX_MAX_KEYS];
+	bool		isnull[INDEX_MAX_KEYS];
+	int			i;
+	HeapTuple htuple;
+
+	for (i = 0; i < index->rd_att->natts; i++)
+	{
+		fetchatt[i] = index_getattr(scandesc->xs_itup, i + 1,
+									scandesc->xs_itupdesc, &isnull[i]);
+	}
+
+	htuple = heap_form_tuple(scandesc->xs_hitupdesc, fetchatt, isnull);
+	htuple->t_tableOid = scandesc->heapRelation->rd_id;
+
+
+	return htuple;
+}
 
 /*
  *	_bt_drop_lock_and_maybe_pin()
@@ -1105,8 +1131,32 @@ readcomplete:
 	/* OK, itemIndex says what to return */
 	currItem = &so->currPos.items[so->currPos.itemIndex];
 	scan->xs_ctup.t_self = currItem->heapTid;
+
 	if (scan->xs_want_itup)
+	{
+		if (!scan->xs_hitupdesc)
+		{
+			int natts = RelationGetNumberOfAttributes(scan->indexRelation);
+			int attno;
+			/*
+			* The storage type of the index can be different from the original
+			* datatype being indexed, so we cannot just grab the index's tuple
+			* descriptor. Instead, construct a descriptor with the original data
+			* types.
+			*/
+			scan->xs_hitupdesc = CreateTemplateTupleDesc(natts, false);
+			for (attno = 1; attno <= natts; attno++)
+			{
+				TupleDescInitEntry(scan->xs_hitupdesc, attno, NULL,
+								scan->indexRelation->rd_opcintype[attno - 1],
+								-1, 0);
+			}
+		}
+
 		scan->xs_itup = (IndexTuple) (so->currTuples + currItem->tupleOffset);
+		scan->xs_hitup = _bt_fetch_tuple(scan);
+		scan->xs_hitup->t_self = currItem->heapTid;
+	}
 
 	return true;
 }
@@ -1155,8 +1205,34 @@ _bt_next(IndexScanDesc scan, ScanDirection dir)
 	/* OK, itemIndex says what to return */
 	currItem = &so->currPos.items[so->currPos.itemIndex];
 	scan->xs_ctup.t_self = currItem->heapTid;
+
 	if (scan->xs_want_itup)
+	{
+		if (!scan->xs_hitupdesc)
+		{
+			int natts = RelationGetNumberOfAttributes(scan->indexRelation);
+			int attno;
+			/*
+			* The storage type of the index can be different from the original
+			* datatype being indexed, so we cannot just grab the index's tuple
+			* descriptor. Instead, construct a descriptor with the original data
+			* types.
+			*/
+			scan->xs_hitupdesc = CreateTemplateTupleDesc(natts, false);
+			for (attno = 1; attno <= natts; attno++)
+			{
+				TupleDescInitEntry(scan->xs_hitupdesc, attno, NULL,
+								scan->indexRelation->rd_opcintype[attno - 1],
+								-1, 0);
+			}
+		}
+
 		scan->xs_itup = (IndexTuple) (so->currTuples + currItem->tupleOffset);
+		if (scan->xs_hitup)
+			pfree(scan->xs_hitup);
+		scan->xs_hitup = _bt_fetch_tuple(scan);
+		scan->xs_hitup->t_self = currItem->heapTid;
+	}
 
 	return true;
 }
@@ -1932,8 +2008,34 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
 	/* OK, itemIndex says what to return */
 	currItem = &so->currPos.items[so->currPos.itemIndex];
 	scan->xs_ctup.t_self = currItem->heapTid;
+
 	if (scan->xs_want_itup)
+	{
+		if (!scan->xs_hitupdesc)
+		{
+			int natts = RelationGetNumberOfAttributes(scan->indexRelation);
+			int attno;
+			/*
+			* The storage type of the index can be different from the original
+			* datatype being indexed, so we cannot just grab the index's tuple
+			* descriptor. Instead, construct a descriptor with the original data
+			* types.
+			*/
+			scan->xs_hitupdesc = CreateTemplateTupleDesc(natts, false);
+			for (attno = 1; attno <= natts; attno++)
+			{
+				TupleDescInitEntry(scan->xs_hitupdesc, attno, NULL,
+								scan->indexRelation->rd_opcintype[attno - 1],
+								-1, 0);
+			}
+		}
+
 		scan->xs_itup = (IndexTuple) (so->currTuples + currItem->tupleOffset);
+		if (scan->xs_hitup)
+			pfree(scan->xs_hitup);
+		scan->xs_hitup = _bt_fetch_tuple(scan);
+		scan->xs_hitup->t_self = currItem->heapTid;
+	}
 
 	return true;
 }
 
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