From 303a4dd46705aa9aef49d0cc92ed66a49c16cd79 Mon Sep 17 00:00:00 2001
From: Matthias van de Meent <boekewurm+postgres@gmail.com>
Date: Fri, 8 Apr 2022 14:51:01 +0200
Subject: [PATCH v5 5/8] Add a function whose task it is to populate all
 attcacheoff-s of a TupleDesc's attributes

It fills uncacheable offsets with -2; as opposed to -1 which signals
"unknown", thus allowing users of the API to determine the cache-ability
of an attribute at O(1) complexity after this one-time O(n) cost, as
opposed to the repeated O(n) cost that currently applies.
---
 src/backend/access/common/tupdesc.c | 97 +++++++++++++++++++++++++++++
 src/include/access/tupdesc.h        |  2 +
 2 files changed, 99 insertions(+)

diff --git a/src/backend/access/common/tupdesc.c b/src/backend/access/common/tupdesc.c
index d6fb261e20..af4ef00f2b 100644
--- a/src/backend/access/common/tupdesc.c
+++ b/src/backend/access/common/tupdesc.c
@@ -919,3 +919,100 @@ BuildDescFromLists(List *names, List *types, List *typmods, List *collations)
 
 	return desc;
 }
+
+/*
+ * PopulateTupleDescCacheOffsets
+ *
+ * Populate the attcacheoff fields of a TupleDesc, returning the last
+ * attcacheoff with a valid value.
+ *
+ * Sets attcacheoff to -2 for uncacheable attributes (i.e. attributes after a
+ * variable length attributes).
+ */
+AttrNumber
+PopulateTupleDescCacheOffsets(TupleDesc desc)
+{
+	int			numberOfAttributes = desc->natts;
+	AttrNumber	i, j;
+
+	if (TupleDescAttr(desc, desc->natts - 1)->attcacheoff != -1)
+	{
+		/*
+		 * Already done the calculations, find the last attribute that has
+		 * cache offset.
+		 */
+		for (i = (AttrNumber) numberOfAttributes; i > 1; i--)
+		{
+			if (TupleDescAttr(desc, i - 1)->attcacheoff != -2)
+				return i;
+		}
+
+		return 1;
+	}
+
+	/*
+	 * First attribute always starts at offset zero.
+	 */
+	TupleDescAttr(desc, 0)->attcacheoff = 0;
+
+	i = 1;
+	/*
+	 * Someone might have set some offsets previously.
+	 * Skip all positive offsets to get to the first attribute without
+	 * attcacheoff.
+	 */
+	while (i < numberOfAttributes && TupleDescAttr(desc, i)->attcacheoff > 0)
+		i++;
+
+	/* Cache offset is undetermined. Start calculating offsets if possible */
+	if (i < numberOfAttributes &&
+		TupleDescAttr(desc, i)->attcacheoff == -1)
+	{
+		Form_pg_attribute att = TupleDescAttr(desc, i - 1);
+		Size off = att->attcacheoff;
+
+		if (att->attlen >= 0) {
+			off += att->attlen;
+
+			while (i < numberOfAttributes)
+			{
+				att = TupleDescAttr(desc, i);
+
+				if (att->attlen < 0)
+				{
+					if (off == att_align_nominal(off, att->attalign))
+						att->attcacheoff = off;
+					else
+						att->attcacheoff = -2;
+					i++;
+					break;
+				}
+
+				off = att_align_nominal(off, att->attalign);
+				att->attcacheoff = off;
+				off += att->attlen;
+				i++;
+			}
+		} else {
+			if (off == att_align_nominal(off, att->attalign))
+				att->attcacheoff = off;
+			else
+				att->attcacheoff = -2;
+			i++;
+		}
+	}
+
+	/*
+	 * No cacheable offsets left. Fill the rest with -2s, but return the latest
+	 * cached offset.
+	 */
+	j = i;
+
+	while (i < numberOfAttributes)
+	{
+		TupleDescAttr(desc, i)->attcacheoff = -2;
+		i++;
+	}
+
+	return j;
+}
diff --git a/src/include/access/tupdesc.h b/src/include/access/tupdesc.h
index 28dd6de18b..219f837875 100644
--- a/src/include/access/tupdesc.h
+++ b/src/include/access/tupdesc.h
@@ -151,4 +151,6 @@ extern TupleDesc BuildDescForRelation(List *schema);
 
 extern TupleDesc BuildDescFromLists(List *names, List *types, List *typmods, List *collations);
 
+extern AttrNumber PopulateTupleDescCacheOffsets(TupleDesc desc);
+
 #endif							/* TUPDESC_H */
-- 
2.30.2