Fix COPY TO FORMAT JSON to exclude generated columns.

COPY TO with FORMAT json was including generated columns in the
output, unlike TEXT and CSV formats.  Virtual generated columns
appeared as null, and stored ones showed their computed values.

The JSON code path only built a restricted TupleDesc when an explicit
column list was given (attnamelist != NIL), but CopyGetAttnums()
also excludes generated columns from the default list.  Fix by
checking whether the attnumlist is shorter than the full TupleDesc
instead.

Bug introduced in 7dadd38cda.

Author: Satya Narlapuram <satya.narlapuram@gmail.com>
Reviewed-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CAHg+QDcfpGDoPL3fvfjXRtfn=fny6DdJR6BAy6TpS1Xj2EZfXA@mail.gmail.com

src/backend/commands/copyto.c

@@ -1033,7 +1033,7 @@ BeginCopyTo(ParseState *pstate,
 	{
 		cstate->json_buf = makeStringInfo();
 
-		if (attnamelist != NIL && rel)
+		if (rel && list_length(cstate->attnumlist) < tupDesc->natts)
 		{
 			int			natts = list_length(cstate->attnumlist);
 			TupleDesc	resultDesc;

src/test/regress/expected/generated_stored.out

@@ -541,6 +541,12 @@ SELECT * FROM gtest3 ORDER BY a;
  4 | 12
 (4 rows)
 
+-- COPY JSON should exclude generated columns, same as text/CSV
+COPY gtest1 TO stdout WITH (FORMAT json);
+{"a":1}
+{"a":2}
+{"a":3}
+{"a":4}
 -- null values
 CREATE TABLE gtest2 (a int PRIMARY KEY, b int GENERATED ALWAYS AS (NULL) STORED);
 INSERT INTO gtest2 VALUES (1);

src/test/regress/expected/generated_virtual.out

@@ -535,6 +535,12 @@ SELECT * FROM gtest3 ORDER BY a;
  4 | 12
 (4 rows)
 
+-- COPY JSON should exclude generated columns, same as text/CSV
+COPY gtest1 TO stdout WITH (FORMAT json);
+{"a":1}
+{"a":2}
+{"a":3}
+{"a":4}
 -- null values
 CREATE TABLE gtest2 (a int PRIMARY KEY, b int GENERATED ALWAYS AS (NULL) VIRTUAL);
 INSERT INTO gtest2 VALUES (1);

src/test/regress/sql/generated_stored.sql

@@ -239,6 +239,9 @@ COPY gtest3 (a, b) FROM stdin;
 
 SELECT * FROM gtest3 ORDER BY a;
 
+-- COPY JSON should exclude generated columns, same as text/CSV
+COPY gtest1 TO stdout WITH (FORMAT json);
+
 -- null values
 CREATE TABLE gtest2 (a int PRIMARY KEY, b int GENERATED ALWAYS AS (NULL) STORED);
 INSERT INTO gtest2 VALUES (1);

src/test/regress/sql/generated_virtual.sql

@@ -239,6 +239,9 @@ COPY gtest3 (a, b) FROM stdin;
 
 SELECT * FROM gtest3 ORDER BY a;
 
+-- COPY JSON should exclude generated columns, same as text/CSV
+COPY gtest1 TO stdout WITH (FORMAT json);
+
 -- null values
 CREATE TABLE gtest2 (a int PRIMARY KEY, b int GENERATED ALWAYS AS (NULL) VIRTUAL);
 INSERT INTO gtest2 VALUES (1);