From 50dad0d8cac934e13538b5294b57e3eb3019d97a Mon Sep 17 00:00:00 2001
From: Drew Kimball <drewk@cockroachlabs.com>
Date: Fri, 11 Jun 2021 17:14:17 -0700
Subject: [PATCH] colexec: call SetLength on output batches for window
 functions

Previously, `bufferedWindowOp` would return batches immediately
after their output columns were entirely filled. However, this caused
the non-decreasing invariant to be violated for bytes output columns
when there were trailing nulls.

This patch modifies `bufferedWindowOp` to call `SetLength` on each
batch before it is returned. This ensures that the bytes invariant is
maintained when there are trailing nulls.

See #64793

Release note: None
---
 pkg/sql/colexec/colexecwindow/buffered_window.go |  8 +++++++-
 pkg/sql/logictest/testdata/logic_test/window     | 16 ++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/pkg/sql/colexec/colexecwindow/buffered_window.go b/pkg/sql/colexec/colexecwindow/buffered_window.go
index 248959c921f2..67233a2723e0 100644
--- a/pkg/sql/colexec/colexecwindow/buffered_window.go
+++ b/pkg/sql/colexec/colexecwindow/buffered_window.go
@@ -310,12 +310,18 @@ func (b *bufferedWindowOp) Next() coldata.Batch {
 					// first of the tuples yet to be processed was somewhere in the batch.
 					b.processingIdx = 0
 				}
+				// Although we didn't change the length of the batch, it is necessary to
+				// set the length anyway (to maintain the invariant of flat bytes).
+				output.SetLength(output.Length())
 				return output
 			}
 			if b.currentBatch.Length() > 0 {
 				b.windower.processBatch(b.currentBatch, b.processingIdx, b.nextPartitionIdx)
 				if b.nextPartitionIdx >= b.currentBatch.Length() {
-					// This was the last batch and it has been entirely filled.
+					// This was the last batch and it has been entirely filled. Although
+					// we didn't change the length of the batch, it is necessary to set
+					// the length anyway (to maintain the invariant of flat bytes).
+					b.currentBatch.SetLength(b.currentBatch.Length())
 					b.state = windowFinished
 					return b.currentBatch
 				}
diff --git a/pkg/sql/logictest/testdata/logic_test/window b/pkg/sql/logictest/testdata/logic_test/window
index 20a57586374b..d8be480e62a5 100644
--- a/pkg/sql/logictest/testdata/logic_test/window
+++ b/pkg/sql/logictest/testdata/logic_test/window
@@ -3988,3 +3988,19 @@ ORDER BY x
 88.0 7527.842222222222222222222 836.42691358024691358 940.98027777777777778 28.921046204801217626 30.675401835636607970
 88.0 7527.842222222222222222222 836.42691358024691358 940.98027777777777778 28.921046204801217626 30.675401835636607970
 89.0 8885.844                   888.5844              987.316               29.809132828715430446 31.421584937746218024
+
+# Regression test for #64793. The output bytes column should not have decreasing
+# offsets.
+statement ok
+CREATE TABLE t64793 (b TEXT)
+
+statement ok
+INSERT INTO t64793 VALUES
+('alpha'),
+(NULL::TEXT)
+
+query T
+SELECT lag(b, 0) OVER (ORDER BY b DESC) FROM t64793 ORDER BY b
+----
+NULL
+alpha