simplify index tuple descriptor initialization

From 7a8d8bd13fd1cdaf18de21f4d45df4269aa71d10 Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter_e@gmx.net>
Date: Mon, 27 Aug 2018 15:50:50 +0200
Subject: [PATCH] Simplify index tuple descriptor initialization

We have two code paths for initializing the tuple descriptor for a new
index: For a normal index, we copy the tuple descriptor from the table
and reset a number of fields that are not applicable to indexes.  For an
expression index, we make a blank tuple descriptor and fill in the
needed fields based on the provided expressions.  As pg_attribute has
grown over time, the number of fields that we need to reset in the first
case is now bigger than the number of fields we actually want to copy,
so it's sensible to do it the other way around: Make a blank descriptor
and copy just the fields we need.  This also allows more code sharing
between the two branches, and it avoids having to touch this code for
almost every unrelated change to the pg_attribute structure.
---
 src/backend/catalog/index.c | 57 +++++++++++++++----------------------
 1 file changed, 23 insertions(+), 34 deletions(-)

diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index b256054908..636fe9c740 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -319,9 +319,7 @@ ConstructTupleDescriptor(Relation heapRelation,
 	indexTupDesc = CreateTemplateTupleDesc(numatts, false);
 
 	/*
-	 * For simple index columns, we copy the pg_attribute row from the parent
-	 * relation and modify it as necessary.  For expressions we have to cons
-	 * up a pg_attribute row the hard way.
+	 * Fill in the pg_attribute row.
 	 */
 	for (i = 0; i < numatts; i++)
 	{
@@ -332,6 +330,19 @@ ConstructTupleDescriptor(Relation heapRelation,
 		Form_pg_opclass opclassTup;
 		Oid			keyType;
 
+		MemSet(to, 0, ATTRIBUTE_FIXED_PART_SIZE);
+		to->attnum = i + 1;
+		to->attstattarget = -1;
+		to->attcacheoff = -1;
+		to->attislocal = true;
+		to->attcollation = (i < numkeyatts) ?
+			collationObjectId[i] : InvalidOid;
+
+		/*
+		 * For simple index columns, we copy some pg_attribute fields from the
+		 * parent relation.  For expressions we have to look at the expression
+		 * result.
+		 */
 		if (atnum != 0)
 		{
 			/* Simple index column */
@@ -356,36 +367,20 @@ ConstructTupleDescriptor(Relation heapRelation,
 									 AttrNumberGetAttrOffset(atnum));
 			}
 
-			/*
-			 * now that we've determined the "from", let's copy the tuple desc
-			 * data...
-			 */
-			memcpy(to, from, ATTRIBUTE_FIXED_PART_SIZE);
-
-			/*
-			 * Fix the stuff that should not be the same as the underlying
-			 * attr
-			 */
-			to->attnum = i + 1;
-
-			to->attstattarget = -1;
-			to->attcacheoff = -1;
-			to->attnotnull = false;
-			to->atthasdef = false;
-			to->atthasmissing = false;
-			to->attidentity = '\0';
-			to->attislocal = true;
-			to->attinhcount = 0;
-			to->attcollation = (i < numkeyatts) ?
-				collationObjectId[i] : InvalidOid;
+			namecpy(&to->attname, &from->attname);
+			to->atttypid = from->atttypid;
+			to->attlen = from->attlen;
+			to->attndims = from->attndims;
+			to->atttypmod = from->atttypmod;
+			to->attbyval = from->attbyval;
+			to->attstorage = from->attstorage;
+			to->attalign = from->attalign;
 		}
 		else
 		{
 			/* Expressional index */
 			Node	   *indexkey;
 
-			MemSet(to, 0, ATTRIBUTE_FIXED_PART_SIZE);
-
 			if (indexpr_item == NULL)	/* shouldn't happen */
 				elog(ERROR, "too few entries in indexprs list");
 			indexkey = (Node *) lfirst(indexpr_item);
@@ -401,20 +396,14 @@ ConstructTupleDescriptor(Relation heapRelation,
 			typeTup = (Form_pg_type) GETSTRUCT(tuple);
 
 			/*
-			 * Assign some of the attributes values. Leave the rest as 0.
+			 * Assign some of the attributes values. Leave the rest.
 			 */
-			to->attnum = i + 1;
 			to->atttypid = keyType;
 			to->attlen = typeTup->typlen;
 			to->attbyval = typeTup->typbyval;
 			to->attstorage = typeTup->typstorage;
 			to->attalign = typeTup->typalign;
-			to->attstattarget = -1;
-			to->attcacheoff = -1;
 			to->atttypmod = exprTypmod(indexkey);
-			to->attislocal = true;
-			to->attcollation = (i < numkeyatts) ?
-				collationObjectId[i] : InvalidOid;
 
 			ReleaseSysCache(tuple);
 
-- 
2.18.0