sql: stop observing the CommitTimestamp in TRUNCATE

In #40581 we stopped observing the commit timestamp to write it into table
descriptors. In this change I overlooked (rather forgot) about this additional
place in the code where we observed the commit timestamp. As far as I can tell
we don't read this field anywhere ever. Furthermore we know that the the table
descriptor in question to which we are referring must be alive and equal to
the provided value at the timestamp at which it was read due to
serializability. In short, this minor change continues to populate the field
with a sensible value and will permit TRUNCATE to be pushed.

Fixes #41566.

Release note (bug fix): Long running transactions which attempt to TRUNCATE
can now be pushed and will commit in cases where they previously could fail
or retry forever.

pkg/sql/sqlbase/structured.proto

@@ -799,6 +799,12 @@ message TableDescriptor {
   message Replacement {
     option (gogoproto.equal) = true;
     optional uint32 id = 1 [(gogoproto.nullable) = false, (gogoproto.customname) = "ID", (gogoproto.casttype) = "ID"];
+    // Time is just used for debugging purposes. It is not used in business
+    // logic. It is an HLC rather than just wall time only for historical
+    // reasons. Prior to 20.1 it was populated with the commit timestamp of the
+    // transaction which created this replacement. In 20.1 and after it is
+    // populated with the read timestamp at which the descriptor being
+    // replaced was read.
     optional util.hlc.Timestamp time = 2 [(gogoproto.nullable) = false];
   }
 

pkg/sql/truncate.go

@@ -190,7 +190,10 @@ func (p *planner) truncateTable(
 	tableDesc.DropJobID = dropJobID
 	newTableDesc := sqlbase.NewMutableCreatedTableDescriptor(tableDesc.TableDescriptor)
 	newTableDesc.ReplacementOf = sqlbase.TableDescriptor_Replacement{
-		ID: id, Time: p.txn.CommitTimestamp(),
+		ID: id,
+		// NB: Time is just used for debugging purposes. See the comment on the
+		// field for more details.
+		Time: p.txn.ReadTimestamp(),
 	}
 	newTableDesc.SetID(0)
 	newTableDesc.Version = 1