Add strict_multi_assignment and too_many_rows plpgsql checks

Until now shadowed_variables was the only plpgsql check supported by
plpgsql.extra_warnings and plpgsql.extra_errors.  This patch introduces
two new checks - strict_multi_assignment and too_many_rows.  Unlike
shadowed_variables, these new checks are enforced at run-time.

strict_multi_assignment checks that commands allowing multi-assignment
(for example SELECT INTO) have the same number of sources and targets.
too_many_rows checks that queries with an INTO clause return one row
exactly.

These checks are aimed at cases that are technically valid and allowed,
but are often a sign of a bug.  Therefore those checks are expected to
be enabled primarily in development and testing environments.

Author: Pavel Stehule
Reviewed-by: Stephen Frost, Tomas Vondra
Discussion: https://www.postgresql.org/message-id/flat/CAFj8pRA2kKRDKpUNwLY0GeG1OqOp+tLS2yQA1V41gzuSz-hCng@mail.gmail.com

doc/src/sgml/plpgsql.sgml

@@ -5034,7 +5034,7 @@ a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like '$$
 
   </sect2>
   <sect2 id="plpgsql-extra-checks">
-   <title>Additional Compile-time Checks</title>
+   <title>Additional Compile-time and Run-time Checks</title>
 
    <para>
     To aid the user in finding instances of simple but common problems before
@@ -5046,13 +5046,19 @@ a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like '$$
     so you are advised to test in a separate development environment.
    </para>
 
+   <para>
+    Setting <varname>plpgsql.extra_warnings</varname>, or
+    <varname>plpgsql.extra_errors</varname>, as appropriate, to <literal>"all"</literal>
+    is encouraged in development and/or testing environments.
+   </para>
+
    <para>
     These additional checks are enabled through the configuration variables
     <varname>plpgsql.extra_warnings</varname> for warnings and
     <varname>plpgsql.extra_errors</varname> for errors. Both can be set either to
-  a comma-separated list of checks, <literal>"none"</literal> or <literal>"all"</literal>.
+    a comma-separated list of checks, <literal>"none"</literal> or
-  The default is <literal>"none"</literal>. Currently the list of available checks
+    <literal>"all"</literal>. The default is <literal>"none"</literal>. Currently
-  includes only one:
+    the list of available checks includes:
     <variablelist>
      <varlistentry>
       <term><varname>shadowed_variables</varname></term>
@@ -5062,6 +5068,38 @@ a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like '$$
        </para>
       </listitem>
      </varlistentry>
+
+     <varlistentry>
+      <term><varname>strict_multi_assignment</varname></term>
+      <listitem>
+       <para>
+        Some <application>PL/PgSQL</application> commands allow assigning
+        values to more than one variable at a time, such as
+        <command>SELECT INTO</command>.  Typically, the number of target
+        variables and the number of source variables should match, though
+        <application>PL/PgSQL</application> will use <literal>NULL</literal>
+        for missing values and extra variables are ignored.  Enabling this
+        check will cause <application>PL/PgSQL</application> to throw a
+        <literal>WARNING</literal> or <literal>ERROR</literal> whenever the
+        number of target variables and the number of source variables are
+        different.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term><varname>too_many_rows</varname></term>
+      <listitem>
+       <para>
+        Enabling this check will cause <application>PL/PgSQL</application> to
+        check if a given query returns more than one row when an
+        <literal>INTO</literal> clause is used.  As an <literal>INTO</literal>
+        statement will only ever use one row, having a query return multiple
+        rows is generally either inefficient and/or nondeterministic and
+        therefore is likely an error.
+       </para>
+      </listitem>
+     </varlistentry>
     </variablelist>
 
     The following example shows the effect of <varname>plpgsql.extra_warnings</varname>
@@ -5080,6 +5118,39 @@ WARNING:  variable "f1" shadows a previously defined variable
 LINE 3: f1 int;
         ^
 CREATE FUNCTION
+</programlisting>
+    The below example shows the effects of setting
+    <varname>plpgsql.extra_warnings</varname> to
+    <varname>strict_multi_assignment</varname>:
+<programlisting>
+SET plpgsql.extra_warnings TO 'strict_multi_assignment';
+
+CREATE OR REPLACE FUNCTION public.foo()
+ RETURNS void
+ LANGUAGE plpgsql
+AS $$
+DECLARE
+  x int;
+  y int;
+BEGIN
+  SELECT 1 INTO x, y;
+  SELECT 1, 2 INTO x, y;
+  SELECT 1, 2, 3 INTO x, y;
+END;
+$$;
+
+SELECT foo();
+WARNING:  number of source and target fields in assignment do not match
+DETAIL:  strict_multi_assignment check of extra_warnings is active.
+HINT:  Make sure the query returns the exact list of columns.
+WARNING:  number of source and target fields in assignment do not match
+DETAIL:  strict_multi_assignment check of extra_warnings is active.
+HINT:  Make sure the query returns the exact list of columns.
+
+ foo 
+-----
+ 
+(1 row)
 </programlisting>
    </para>
   </sect2>

src/pl/plpgsql/src/pl_exec.c

@@ -4020,6 +4020,12 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 	long		tcount;
 	int			rc;
 	PLpgSQL_expr *expr = stmt->sqlstmt;
+	int			too_many_rows_level = 0;
+
+	if (plpgsql_extra_errors & PLPGSQL_XCHECK_TOOMANYROWS)
+		too_many_rows_level = ERROR;
+	else if (plpgsql_extra_warnings & PLPGSQL_XCHECK_TOOMANYROWS)
+		too_many_rows_level = WARNING;
 
 	/*
 	 * On the first call for this statement generate the plan, and detect
@@ -4059,9 +4065,10 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 
 	/*
 	 * If we have INTO, then we only need one row back ... but if we have INTO
-	 * STRICT, ask for two rows, so that we can verify the statement returns
+	 * STRICT or extra check too_many_rows, ask for two rows, so that we can
-	 * only one.  INSERT/UPDATE/DELETE are always treated strictly. Without
+	 * verify the statement returns only one.  INSERT/UPDATE/DELETE are always
-	 * INTO, just run the statement to completion (tcount = 0).
+	 * treated strictly. Without INTO, just run the statement to completion
+	 * (tcount = 0).
 	 *
 	 * We could just ask for two rows always when using INTO, but there are
 	 * some cases where demanding the extra row costs significant time, eg by
@@ -4070,7 +4077,7 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 	 */
 	if (stmt->into)
 	{
-		if (stmt->strict || stmt->mod_stmt)
+		if (stmt->strict || stmt->mod_stmt || too_many_rows_level)
 			tcount = 2;
 		else
 			tcount = 1;
@@ -4187,19 +4194,23 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 		}
 		else
 		{
-			if (n > 1 && (stmt->strict || stmt->mod_stmt))
+			if (n > 1 && (stmt->strict || stmt->mod_stmt || too_many_rows_level))
 			{
 				char	   *errdetail;
+				int			errlevel;
 
 				if (estate->func->print_strict_params)
 					errdetail = format_expr_params(estate, expr);
 				else
 					errdetail = NULL;
 
-				ereport(ERROR,
+				errlevel = (stmt->strict || stmt->mod_stmt) ? ERROR : too_many_rows_level;
+
+				ereport(errlevel,
 						(errcode(ERRCODE_TOO_MANY_ROWS),
 						 errmsg("query returned more than one row"),
-						 errdetail ? errdetail_internal("parameters: %s", errdetail) : 0));
+						 errdetail ? errdetail_internal("parameters: %s", errdetail) : 0,
+						 errhint("Make sure the query returns a single row, or use LIMIT 1")));
 			}
 			/* Put the first result row into the target */
 			exec_move_row(estate, target, tuptab->vals[0], tuptab->tupdesc);
@@ -6835,6 +6846,19 @@ exec_move_row_from_fields(PLpgSQL_execstate *estate,
 	int			td_natts = tupdesc ? tupdesc->natts : 0;
 	int			fnum;
 	int			anum;
+	int			strict_multiassignment_level = 0;
+
+	/*
+	 * The extra check strict strict_multi_assignment can be active,
+	 * only when input tupdesc is specified.
+	 */
+	if (tupdesc != NULL)
+	{
+		if (plpgsql_extra_errors & PLPGSQL_XCHECK_STRICTMULTIASSIGNMENT)
+			strict_multiassignment_level = ERROR;
+		else if (plpgsql_extra_warnings & PLPGSQL_XCHECK_STRICTMULTIASSIGNMENT)
+			strict_multiassignment_level = WARNING;
+	}
 
 	/* Handle RECORD-target case */
 	if (target->dtype == PLPGSQL_DTYPE_REC)
@@ -6913,10 +6937,23 @@ exec_move_row_from_fields(PLpgSQL_execstate *estate,
 				}
 				else
 				{
+					/* no source for destination column */
 					value = (Datum) 0;
 					isnull = true;
 					valtype = UNKNOWNOID;
 					valtypmod = -1;
+
+					/* When source value is missing */
+					if (strict_multiassignment_level)
+							ereport(strict_multiassignment_level,
+									(errcode(ERRCODE_DATATYPE_MISMATCH),
+									 errmsg("number of source and target fields in assignment do not match"),
+									 /* translator: %s represents a name of an extra check */
+									 errdetail("%s check of %s is active.",
+											   "strict_multi_assignment",
+													  strict_multiassignment_level == ERROR ? "extra_errors" :
+																	  "extra_warnings"),
+									 errhint("Make sure the query returns the exact list of columns.")));
 				}
 
 				/* Cast the new value to the right type, if needed. */
@@ -6930,6 +6967,29 @@ exec_move_row_from_fields(PLpgSQL_execstate *estate,
 				newnulls[fnum] = isnull;
 			}
 
+			/*
+			 * When strict_multiassignment extra check is active, then ensure
+			 * there are no unassigned source attributes.
+			 */
+			if (strict_multiassignment_level && anum < td_natts)
+			{
+				/* skip dropped columns in the source descriptor */
+				while (anum < td_natts &&
+					   TupleDescAttr(tupdesc, anum)->attisdropped)
+					anum++;
+
+				if (anum < td_natts)
+					ereport(strict_multiassignment_level,
+							(errcode(ERRCODE_DATATYPE_MISMATCH),
+							 errmsg("number of source and target fields in assignment do not match"),
+							 /* translator: %s represents a name of an extra check */
+							 errdetail("%s check of %s is active.",
+									   "strict_multi_assignment",
+										  strict_multiassignment_level == ERROR ? "extra_errors" :
+																	  "extra_warnings"),
+							 errhint("Make sure the query returns the exact list of columns.")));
+			}
+
 			values = newvalues;
 			nulls = newnulls;
 		}
@@ -6986,16 +7046,50 @@ exec_move_row_from_fields(PLpgSQL_execstate *estate,
 			}
 			else
 			{
+				/* no source for destination column */
 				value = (Datum) 0;
 				isnull = true;
 				valtype = UNKNOWNOID;
 				valtypmod = -1;
+
+				if (strict_multiassignment_level)
+						ereport(strict_multiassignment_level,
+								(errcode(ERRCODE_DATATYPE_MISMATCH),
+								 errmsg("number of source and target fields in assignment do not match"),
+						 /* translator: %s represents a name of an extra check */
+						 errdetail("%s check of %s is active.",
+								  "strict_multi_assignment",
+								  strict_multiassignment_level == ERROR ? "extra_errors" :
+																	  "extra_warnings"),
+								 errhint("Make sure the query returns the exact list of columns.")));
 			}
 
 			exec_assign_value(estate, (PLpgSQL_datum *) var,
 							  value, isnull, valtype, valtypmod);
 		}
 
+		/*
+		 * When strict_multiassignment extra check is active, ensure there
+		 * are no unassigned source attributes.
+		 */
+		if (strict_multiassignment_level && anum < td_natts)
+		{
+			while (anum < td_natts &&
+				   TupleDescAttr(tupdesc, anum)->attisdropped)
+				anum++;			/* skip dropped column in tuple */
+
+			if (anum < td_natts)
+				ereport(strict_multiassignment_level,
+						(errcode(ERRCODE_DATATYPE_MISMATCH),
+						 errmsg("number of source and target fields in assignment do not match"),
+						 /* translator: %s represents a name of an extra check */
+						 errdetail("%s check of %s is active.",
+								  "strict_multi_assignment",
+								  strict_multiassignment_level == ERROR ? "extra_errors" :
+																	  "extra_warnings"),
+						 errhint("Make sure the query returns the exact list of columns.")));
+		}
+
 		return;
 	}
 

src/pl/plpgsql/src/pl_handler.c

@@ -92,6 +92,10 @@ plpgsql_extra_checks_check_hook(char **newvalue, void **extra, GucSource source)
 
 			if (pg_strcasecmp(tok, "shadowed_variables") == 0)
 				extrachecks |= PLPGSQL_XCHECK_SHADOWVAR;
+			else if (pg_strcasecmp(tok, "too_many_rows") == 0)
+				extrachecks |= PLPGSQL_XCHECK_TOOMANYROWS;
+			else if (pg_strcasecmp(tok, "strict_multi_assignment") == 0)
+				extrachecks |= PLPGSQL_XCHECK_STRICTMULTIASSIGNMENT;
 			else if (pg_strcasecmp(tok, "all") == 0 || pg_strcasecmp(tok, "none") == 0)
 			{
 				GUC_check_errdetail("Key word \"%s\" cannot be combined with other key words.", tok);

src/pl/plpgsql/src/plpgsql.h

@@ -1135,9 +1135,11 @@ extern bool plpgsql_print_strict_params;
 
 extern bool plpgsql_check_asserts;
 
-/* extra compile-time checks */
+/* extra compile-time and run-time checks */
 #define PLPGSQL_XCHECK_NONE						0
-#define PLPGSQL_XCHECK_SHADOWVAR	1
+#define PLPGSQL_XCHECK_SHADOWVAR				(1 << 1)
+#define PLPGSQL_XCHECK_TOOMANYROWS				(1 << 2)
+#define PLPGSQL_XCHECK_STRICTMULTIASSIGNMENT	(1 << 3)
 #define PLPGSQL_XCHECK_ALL						((int) ~0)
 
 extern int	plpgsql_extra_warnings;

src/test/regress/expected/plpgsql.out

@@ -2778,6 +2778,7 @@ begin
 end$$ language plpgsql;
 select stricttest();
 ERROR:  query returned more than one row
+HINT:  Make sure the query returns a single row, or use LIMIT 1
 CONTEXT:  PL/pgSQL function stricttest() line 5 at SQL statement
 create or replace function stricttest() returns void as $$
 declare x record;
@@ -2851,6 +2852,7 @@ begin
 end$$ language plpgsql;
 select stricttest();
 ERROR:  query returned more than one row
+HINT:  Make sure the query returns a single row, or use LIMIT 1
 CONTEXT:  PL/pgSQL function stricttest() line 5 at SQL statement
 create or replace function stricttest() returns void as $$
 declare x record;
@@ -2916,6 +2918,7 @@ end$$ language plpgsql;
 select stricttest();
 ERROR:  query returned more than one row
 DETAIL:  parameters: p1 = '2', p3 = 'foo'
+HINT:  Make sure the query returns a single row, or use LIMIT 1
 CONTEXT:  PL/pgSQL function stricttest() line 8 at SQL statement
 create or replace function stricttest() returns void as $$
 declare x record;
@@ -2926,6 +2929,7 @@ begin
 end$$ language plpgsql;
 select stricttest();
 ERROR:  query returned more than one row
+HINT:  Make sure the query returns a single row, or use LIMIT 1
 CONTEXT:  PL/pgSQL function stricttest() line 5 at SQL statement
 create or replace function stricttest() returns void as $$
 declare x record;
@@ -2973,6 +2977,7 @@ begin
 end$$ language plpgsql;
 select stricttest();
 ERROR:  query returned more than one row
+HINT:  Make sure the query returns a single row, or use LIMIT 1
 CONTEXT:  PL/pgSQL function stricttest() line 10 at SQL statement
 reset plpgsql.print_strict_params;
 create or replace function stricttest() returns void as $$
@@ -2990,6 +2995,7 @@ end$$ language plpgsql;
 select stricttest();
 ERROR:  query returned more than one row
 DETAIL:  parameters: p1 = '2', p3 = 'foo'
+HINT:  Make sure the query returns a single row, or use LIMIT 1
 CONTEXT:  PL/pgSQL function stricttest() line 10 at SQL statement
 -- test warnings and errors
 set plpgsql.extra_warnings to 'all';
@@ -3113,6 +3119,107 @@ select shadowtest(1);
  t
 (1 row)
 
+-- runtime extra checks
+set plpgsql.extra_warnings to 'too_many_rows';
+do $$
+declare x int;
+begin
+  select v from generate_series(1,2) g(v) into x;
+end;
+$$;
+WARNING:  query returned more than one row
+HINT:  Make sure the query returns a single row, or use LIMIT 1
+set plpgsql.extra_errors to 'too_many_rows';
+do $$
+declare x int;
+begin
+  select v from generate_series(1,2) g(v) into x;
+end;
+$$;
+ERROR:  query returned more than one row
+HINT:  Make sure the query returns a single row, or use LIMIT 1
+CONTEXT:  PL/pgSQL function inline_code_block line 4 at SQL statement
+reset plpgsql.extra_errors;
+reset plpgsql.extra_warnings;
+set plpgsql.extra_warnings to 'strict_multi_assignment';
+do $$
+declare
+  x int;
+  y int;
+begin
+  select 1 into x, y;
+  select 1,2 into x, y;
+  select 1,2,3 into x, y;
+end
+$$;
+WARNING:  number of source and target fields in assignment do not match
+DETAIL:  strict_multi_assignment check of extra_warnings is active.
+HINT:  Make sure the query returns the exact list of columns.
+WARNING:  number of source and target fields in assignment do not match
+DETAIL:  strict_multi_assignment check of extra_warnings is active.
+HINT:  Make sure the query returns the exact list of columns.
+set plpgsql.extra_errors to 'strict_multi_assignment';
+do $$
+declare
+  x int;
+  y int;
+begin
+  select 1 into x, y;
+  select 1,2 into x, y;
+  select 1,2,3 into x, y;
+end
+$$;
+ERROR:  number of source and target fields in assignment do not match
+DETAIL:  strict_multi_assignment check of extra_errors is active.
+HINT:  Make sure the query returns the exact list of columns.
+CONTEXT:  PL/pgSQL function inline_code_block line 6 at SQL statement
+create table test_01(a int, b int, c int);
+alter table test_01 drop column a;
+-- the check is active only when source table is not empty
+insert into test_01 values(10,20);
+do $$
+declare
+  x int;
+  y int;
+begin
+  select * from test_01 into x, y; -- should be ok
+  raise notice 'ok';
+  select * from test_01 into x;    -- should to fail
+end;
+$$;
+NOTICE:  ok
+ERROR:  number of source and target fields in assignment do not match
+DETAIL:  strict_multi_assignment check of extra_errors is active.
+HINT:  Make sure the query returns the exact list of columns.
+CONTEXT:  PL/pgSQL function inline_code_block line 8 at SQL statement
+do $$
+declare
+  t test_01;
+begin
+  select 1, 2 into t;  -- should be ok
+  raise notice 'ok';
+  select 1, 2, 3 into t; -- should fail;
+end;
+$$;
+NOTICE:  ok
+ERROR:  number of source and target fields in assignment do not match
+DETAIL:  strict_multi_assignment check of extra_errors is active.
+HINT:  Make sure the query returns the exact list of columns.
+CONTEXT:  PL/pgSQL function inline_code_block line 7 at SQL statement
+do $$
+declare
+  t test_01;
+begin
+  select 1 into t; -- should fail;
+end;
+$$;
+ERROR:  number of source and target fields in assignment do not match
+DETAIL:  strict_multi_assignment check of extra_errors is active.
+HINT:  Make sure the query returns the exact list of columns.
+CONTEXT:  PL/pgSQL function inline_code_block line 5 at SQL statement
+drop table test_01;
+reset plpgsql.extra_errors;
+reset plpgsql.extra_warnings;
 -- test scrollable cursor support
 create function sc_test() returns setof integer as $$
 declare

src/test/regress/sql/plpgsql.sql

@@ -2627,6 +2627,95 @@ declare f1 int; begin return 1; end $$ language plpgsql;
 
 select shadowtest(1);
 
+-- runtime extra checks
+set plpgsql.extra_warnings to 'too_many_rows';
+
+do $$
+declare x int;
+begin
+  select v from generate_series(1,2) g(v) into x;
+end;
+$$;
+
+set plpgsql.extra_errors to 'too_many_rows';
+
+do $$
+declare x int;
+begin
+  select v from generate_series(1,2) g(v) into x;
+end;
+$$;
+
+reset plpgsql.extra_errors;
+reset plpgsql.extra_warnings;
+
+set plpgsql.extra_warnings to 'strict_multi_assignment';
+
+do $$
+declare
+  x int;
+  y int;
+begin
+  select 1 into x, y;
+  select 1,2 into x, y;
+  select 1,2,3 into x, y;
+end
+$$;
+
+set plpgsql.extra_errors to 'strict_multi_assignment';
+
+do $$
+declare
+  x int;
+  y int;
+begin
+  select 1 into x, y;
+  select 1,2 into x, y;
+  select 1,2,3 into x, y;
+end
+$$;
+
+create table test_01(a int, b int, c int);
+
+alter table test_01 drop column a;
+
+-- the check is active only when source table is not empty
+insert into test_01 values(10,20);
+
+do $$
+declare
+  x int;
+  y int;
+begin
+  select * from test_01 into x, y; -- should be ok
+  raise notice 'ok';
+  select * from test_01 into x;    -- should to fail
+end;
+$$;
+
+do $$
+declare
+  t test_01;
+begin
+  select 1, 2 into t;  -- should be ok
+  raise notice 'ok';
+  select 1, 2, 3 into t; -- should fail;
+end;
+$$;
+
+do $$
+declare
+  t test_01;
+begin
+  select 1 into t; -- should fail;
+end;
+$$;
+
+drop table test_01;
+
+reset plpgsql.extra_errors;
+reset plpgsql.extra_warnings;
+
 -- test scrollable cursor support
 
 create function sc_test() returns setof integer as $$