opt: internal error due to outer variable in left side of join

[rytaft]

To reproduce, create a database with the TPC-C schema, then run the following query:

SELECT
	subq_0.c0 AS c0,
	subq_0.c0 AS c1,
	subq_0.c0 AS c2,
	subq_0.c0 AS c3,
	CASE
	WHEN subq_0.c1 IS NOT NULL THEN subq_0.c0
	ELSE subq_0.c0
	END
		AS c4
FROM
	(
		SELECT
			ref_0.h_data AS c0,
			(SELECT i_name FROM item LIMIT 1 OFFSET 6) AS c1
		FROM
			history AS ref_0
			INNER JOIN order_line AS ref_1 ON
					(ref_0.h_data = ref_1.ol_dist_info)
		WHERE
			(
				EXISTS(
					SELECT
						ref_2.h_amount AS c0,
						ref_2.h_c_w_id AS c1,
						ref_2.h_c_d_id AS c2
					FROM
						history AS ref_2
					WHERE
						(ref_2.h_data IS NOT NULL)
						AND (ref_1.ol_dist_info IS NOT NULL)
				)
			)
			OR (
					(SELECT ol_i_id FROM order_line LIMIT 1 OFFSET 6)
					IS NOT NULL
				)
		LIMIT
			2
	)
		AS subq_0
WHERE
	subq_0.c1 IS NULL
LIMIT
	6;

It crashes with this error:

Error: pq: internal error: cannot map variable 19 to an indexed var
DETAIL: stack trace:
github.com/cockroachdb/cockroach/pkg/sql/opt/exec/execbuilder/scalar.go:149: indexedVar()
github.com/cockroachdb/cockroach/pkg/sql/opt/exec/execbuilder/scalar.go:134: buildVariable()
github.com/cockroachdb/cockroach/pkg/sql/opt/exec/execbuilder/scalar.go:91: buildScalar()
github.com/cockroachdb/cockroach/pkg/sql/opt/exec/execbuilder/scalar.go:225: buildComparison()
github.com/cockroachdb/cockroach/pkg/sql/opt/exec/execbuilder/scalar.go:91: buildScalar()
github.com/cockroachdb/cockroach/pkg/sql/opt/exec/execbuilder/scalar.go:212: buildBoolean()
github.com/cockroachdb/cockroach/pkg/sql/opt/exec/execbuilder/scalar.go:91: buildScalar()
...

Thanks to the Apollo team at Georgia Tech for finding this issue.

[rytaft]

Also thanks to @RaduBerinde for narrowing down the root cause and the updates to check_expr.go.