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diff --git a/src/backend/access/common/tupconvert.c b/src/backend/access/common/tupconvert.c
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+++ b/src/backend/access/common/tupconvert.c
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+/*-------------------------------------------------------------------------
+ *
+ * tupconvert.c
+ *	  Tuple conversion support.
+ *
+ * These functions provide conversion between rowtypes that are logically
+ * equivalent but might have columns in a different order or different sets of
+ * dropped columns.
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/access/common/tupconvert.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/tupconvert.h"
+#include "executor/tuptable.h"
+
+
+/*
+ * The conversion setup routines have the following common API:
+ *
+ * The setup routine checks using attmap.c whether the given source and
+ * destination tuple descriptors are logically compatible.  If not, it throws
+ * an error.  If so, it returns NULL if they are physically compatible (ie, no
+ * conversion is needed), else a TupleConversionMap that can be used by
+ * execute_attr_map_tuple or execute_attr_map_slot to perform the conversion.
+ *
+ * The TupleConversionMap, if needed, is palloc'd in the caller's memory
+ * context.  Also, the given tuple descriptors are referenced by the map,
+ * so they must survive as long as the map is needed.
+ *
+ * The caller must supply a suitable primary error message to be used if
+ * a compatibility error is thrown.  Recommended coding practice is to use
+ * gettext_noop() on this string, so that it is translatable but won't
+ * actually be translated unless the error gets thrown.
+ *
+ *
+ * Implementation notes:
+ *
+ * The key component of a TupleConversionMap is an attrMap[] array with
+ * one entry per output column.  This entry contains the 1-based index of
+ * the corresponding input column, or zero to force a NULL value (for
+ * a dropped output column).  The TupleConversionMap also contains workspace
+ * arrays.
+ */
+
+
+/*
+ * Set up for tuple conversion, matching input and output columns by
+ * position.  (Dropped columns are ignored in both input and output.)
+ */
+TupleConversionMap *
+convert_tuples_by_position(TupleDesc indesc,
+						   TupleDesc outdesc,
+						   const char *msg)
+{
+	TupleConversionMap *map;
+	int			n;
+	AttrMap    *attrMap;
+
+	/* Verify compatibility and prepare attribute-number map */
+	attrMap = build_attrmap_by_position(indesc, outdesc, msg);
+
+	if (attrMap == NULL)
+	{
+		/* runtime conversion is not needed */
+		return NULL;
+	}
+
+	/* Prepare the map structure */
+	map = (TupleConversionMap *) palloc(sizeof(TupleConversionMap));
+	map->indesc = indesc;
+	map->outdesc = outdesc;
+	map->attrMap = attrMap;
+	/* preallocate workspace for Datum arrays */
+	n = outdesc->natts + 1;		/* +1 for NULL */
+	map->outvalues = (Datum *) palloc(n * sizeof(Datum));
+	map->outisnull = (bool *) palloc(n * sizeof(bool));
+	n = indesc->natts + 1;		/* +1 for NULL */
+	map->invalues = (Datum *) palloc(n * sizeof(Datum));
+	map->inisnull = (bool *) palloc(n * sizeof(bool));
+	map->invalues[0] = (Datum) 0;	/* set up the NULL entry */
+	map->inisnull[0] = true;
+
+	return map;
+}
+
+/*
+ * Set up for tuple conversion, matching input and output columns by name.
+ * (Dropped columns are ignored in both input and output.)	This is intended
+ * for use when the rowtypes are related by inheritance, so we expect an exact
+ * match of both type and typmod.  The error messages will be a bit unhelpful
+ * unless both rowtypes are named composite types.
+ */
+TupleConversionMap *
+convert_tuples_by_name(TupleDesc indesc,
+					   TupleDesc outdesc)
+{
+	TupleConversionMap *map;
+	AttrMap    *attrMap;
+	int			n = outdesc->natts;
+
+	/* Verify compatibility and prepare attribute-number map */
+	attrMap = build_attrmap_by_name_if_req(indesc, outdesc);
+
+	if (attrMap == NULL)
+	{
+		/* runtime conversion is not needed */
+		return NULL;
+	}
+
+	/* Prepare the map structure */
+	map = (TupleConversionMap *) palloc(sizeof(TupleConversionMap));
+	map->indesc = indesc;
+	map->outdesc = outdesc;
+	map->attrMap = attrMap;
+	/* preallocate workspace for Datum arrays */
+	map->outvalues = (Datum *) palloc(n * sizeof(Datum));
+	map->outisnull = (bool *) palloc(n * sizeof(bool));
+	n = indesc->natts + 1;		/* +1 for NULL */
+	map->invalues = (Datum *) palloc(n * sizeof(Datum));
+	map->inisnull = (bool *) palloc(n * sizeof(bool));
+	map->invalues[0] = (Datum) 0;	/* set up the NULL entry */
+	map->inisnull[0] = true;
+
+	return map;
+}
+
+/*
+ * Perform conversion of a tuple according to the map.
+ */
+HeapTuple
+execute_attr_map_tuple(HeapTuple tuple, TupleConversionMap *map)
+{
+	AttrMap    *attrMap = map->attrMap;
+	Datum	   *invalues = map->invalues;
+	bool	   *inisnull = map->inisnull;
+	Datum	   *outvalues = map->outvalues;
+	bool	   *outisnull = map->outisnull;
+	int			i;
+
+	/*
+	 * Extract all the values of the old tuple, offsetting the arrays so that
+	 * invalues[0] is left NULL and invalues[1] is the first source attribute;
+	 * this exactly matches the numbering convention in attrMap.
+	 */
+	heap_deform_tuple(tuple, map->indesc, invalues + 1, inisnull + 1);
+
+	/*
+	 * Transpose into proper fields of the new tuple.
+	 */
+	Assert(attrMap->maplen == map->outdesc->natts);
+	for (i = 0; i < attrMap->maplen; i++)
+	{
+		int			j = attrMap->attnums[i];
+
+		outvalues[i] = invalues[j];
+		outisnull[i] = inisnull[j];
+	}
+
+	/*
+	 * Now form the new tuple.
+	 */
+	return heap_form_tuple(map->outdesc, outvalues, outisnull);
+}
+
+/*
+ * Perform conversion of a tuple slot according to the map.
+ */
+TupleTableSlot *
+execute_attr_map_slot(AttrMap *attrMap,
+					  TupleTableSlot *in_slot,
+					  TupleTableSlot *out_slot)
+{
+	Datum	   *invalues;
+	bool	   *inisnull;
+	Datum	   *outvalues;
+	bool	   *outisnull;
+	int			outnatts;
+	int			i;
+
+	/* Sanity checks */
+	Assert(in_slot->tts_tupleDescriptor != NULL &&
+		   out_slot->tts_tupleDescriptor != NULL);
+	Assert(in_slot->tts_values != NULL && out_slot->tts_values != NULL);
+
+	outnatts = out_slot->tts_tupleDescriptor->natts;
+
+	/* Extract all the values of the in slot. */
+	slot_getallattrs(in_slot);
+
+	/* Before doing the mapping, clear any old contents from the out slot */
+	ExecClearTuple(out_slot);
+
+	invalues = in_slot->tts_values;
+	inisnull = in_slot->tts_isnull;
+	outvalues = out_slot->tts_values;
+	outisnull = out_slot->tts_isnull;
+
+	/* Transpose into proper fields of the out slot. */
+	for (i = 0; i < outnatts; i++)
+	{
+		int			j = attrMap->attnums[i] - 1;
+
+		/* attrMap->attnums[i] == 0 means it's a NULL datum. */
+		if (j == -1)
+		{
+			outvalues[i] = (Datum) 0;
+			outisnull[i] = true;
+		}
+		else
+		{
+			outvalues[i] = invalues[j];
+			outisnull[i] = inisnull[j];
+		}
+	}
+
+	ExecStoreVirtualTuple(out_slot);
+
+	return out_slot;
+}
+
+/*
+ * Perform conversion of bitmap of columns according to the map.
+ *
+ * The input and output bitmaps are offset by
+ * FirstLowInvalidHeapAttributeNumber to accommodate system cols, like the
+ * column-bitmaps in RangeTblEntry.
+ */
+Bitmapset *
+execute_attr_map_cols(AttrMap *attrMap, Bitmapset *in_cols)
+{
+	Bitmapset *out_cols;
+	int			out_attnum;
+
+	/* fast path for the common trivial case */
+	if (in_cols == NULL)
+		return NULL;
+
+	/*
+	 * For each output column, check which input column it corresponds to.
+	 */
+	out_cols = NULL;
+
+	for (out_attnum = FirstLowInvalidHeapAttributeNumber + 1;
+		 out_attnum <= attrMap->maplen;
+		 out_attnum++)
+	{
+		int			in_attnum;
+
+		if (out_attnum < 0)
+		{
+			/* System column. No mapping. */
+			in_attnum = out_attnum;
+		}
+		else if (out_attnum == 0)
+			continue;
+		else
+		{
+			/* normal user column */
+			in_attnum = attrMap->attnums[out_attnum - 1];
+
+			if (in_attnum == 0)
+				continue;
+		}
+
+		if (bms_is_member(in_attnum - FirstLowInvalidHeapAttributeNumber, in_cols))
+			out_cols = bms_add_member(out_cols, out_attnum - FirstLowInvalidHeapAttributeNumber);
+	}
+
+	return out_cols;
+}
+
+/*
+ * Free a TupleConversionMap structure.
+ */
+void
+free_conversion_map(TupleConversionMap *map)
+{
+	/* indesc and outdesc are not ours to free */
+	free_attrmap(map->attrMap);
+	pfree(map->invalues);
+	pfree(map->inisnull);
+	pfree(map->outvalues);
+	pfree(map->outisnull);
+	pfree(map);
+}