Fix flat select bug

[andyfengHKU]

Description

See PR comments.

Fixes # (issue)

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[github-actions]

Benchmark Result

Master commit hash: 4403706fc2c3a239e0bedbc1deb6668157d3af8a
Branch commit hash: 8ae5969f0b2d314e939c59e6deadef0ea996168e

Query Group	Query Name	Mean Time - Commit (ms)	Mean Time - Master (ms)	Diff
aggregation	q24	643.68	646.05	-2.37 (-0.37%)
aggregation	q28	12058.46	11408.08	650.38 (5.70%)
filter	q14	124.33	125.49	-1.16 (-0.92%)
filter	q15	122.80	126.60	-3.80 (-3.00%)
filter	q16	317.35	304.64	12.71 (4.17%)
filter	q17	444.16	444.60	-0.44 (-0.10%)
filter	q18	1939.00	1951.74	-12.74 (-0.65%)
filter	zonemap-node	86.81	87.03	-0.22 (-0.25%)
filter	zonemap-node-lhs-cast	87.50	86.87	0.63 (0.73%)
filter	zonemap-rel	5571.87	5665.43	-93.56 (-1.65%)
fixed_size_expr_evaluator	q07	579.09	572.59	6.50 (1.13%)
fixed_size_expr_evaluator	q08	807.34	798.75	8.59 (1.08%)
fixed_size_expr_evaluator	q09	808.95	799.39	9.56 (1.20%)
fixed_size_expr_evaluator	q10	244.47	235.88	8.60 (3.64%)
fixed_size_expr_evaluator	q11	237.42	228.71	8.72 (3.81%)
fixed_size_expr_evaluator	q12	232.68	227.73	4.95 (2.18%)
fixed_size_expr_evaluator	q13	1446.75	1451.27	-4.52 (-0.31%)
fixed_size_seq_scan	q23	118.81	107.59	11.22 (10.43%)
join	q29	640.90	606.37	34.52 (5.69%)
join	q30	1400.80	1311.38	89.43 (6.82%)
join	q31	4.53	6.72	-2.19 (-32.58%)
ldbc_snb_ic	q35	396.46	402.93	-6.48 (-1.61%)
ldbc_snb_ic	q36	113.51	88.92	24.59 (27.66%)
ldbc_snb_is	q32	3.05	3.47	-0.42 (-12.02%)
ldbc_snb_is	q33	10.56	11.92	-1.36 (-11.44%)
ldbc_snb_is	q34	1.42	2.04	-0.62 (-30.40%)
multi-rel	multi-rel-large-scan	1192.00	1206.06	-14.06 (-1.17%)
multi-rel	multi-rel-lookup	5.69	32.19	-26.50 (-82.31%)
multi-rel	multi-rel-small-scan	75.63	105.65	-30.02 (-28.41%)
order_by	q25	129.42	128.71	0.71 (0.55%)
order_by	q26	471.99	462.28	9.72 (2.10%)
order_by	q27	1460.47	1472.63	-12.16 (-0.83%)
scan_after_filter	q01	169.45	172.38	-2.92 (-1.69%)
scan_after_filter	q02	154.33	156.90	-2.57 (-1.64%)
shortest_path_ldbc100	q37	81.95	80.79	1.16 (1.44%)
shortest_path_ldbc100	q38	492.35	509.69	-17.34 (-3.40%)
shortest_path_ldbc100	q39	59.31	62.03	-2.71 (-4.38%)
shortest_path_ldbc100	q40	534.97	563.55	-28.58 (-5.07%)
var_size_expr_evaluator	q03	2071.73	2065.13	6.59 (0.32%)
var_size_expr_evaluator	q04	2263.08	2270.34	-7.26 (-0.32%)
var_size_expr_evaluator	q05	2669.28	2596.23	73.06 (2.81%)
var_size_expr_evaluator	q06	1354.08	1349.49	4.59 (0.34%)
var_size_seq_scan	q19	1446.52	1445.19	1.32 (0.09%)
var_size_seq_scan	q20	2575.81	2659.85	-84.04 (-3.16%)
var_size_seq_scan	q21	2277.75	2264.41	13.33 (0.59%)
var_size_seq_scan	q22	125.47	127.17	-1.71 (-1.34%)