Index scan(row count should be less than 8) costs more than table full scan #59133

create table t(a int, b int, key(a));
insert into t values (52, 9),(40, 74),(68, 14),(79, 96),(21, 46),(19, 8),(82, 14),(92, 3),(322, 331),(359, 275),(956, 642),(238, 792),(266, 266),(976, 485),(711, 823),(748, 514),(115, 709),(147, 695),(203, 517),(391, 786),(316, 337),(758, 255),(686, 926),(667, 863),(126, 973),(986, 172),(566, 271),(308, 409),(534, 659),(314, 619),(643, 328),(404, 847),(695, 294),(168, 553),(450, 599),(438, 396),(610, 454),(492, 794),(376, 151),(904, 858),(875, 835),(531, 677),(209, 461),(765, 720),(183, 861),(121, 884),(675, 954),(883, 287),(535, 812),(594, 925),(838, 692),(140, 953),(592, 728),(879, 489),(192, 879),(142, 430),(483, 562),(940, 613),(679, 451),(938, 792),(566, 656),(305, 255),(356, 342),(250, 963),(260, 846),(118, 279),(329, 986),(382, 997),(203, 227),(107, 881),(261, 308),(117, 504),(728, 669),(406, 341),(807, 635),(566, 966),(943, 614),(242, 361),(863, 135),(969, 447),(307, 490),(926, 855),(297, 821),(484, 307),(456, 591),(205, 964),(822, 424),(852, 537),(881, 427),(840, 576),(414, 664),(783, 654),(907, 579),(561, 745),(541, 668),(509, 362),(756, 430),(675, 930),(775, 865),(339, 628),(592, 692),(489, 960),(552, 176),(785, 711),(863, 179),(497, 611),(383, 866),(893, 291),(498, 382),(424, 292),(378, 162),(195, 309),(803, 863),(718, 255),(188, 169),(937, 418),(940, 487),(449, 241),(998, 983),(795, 212),(102, 942),(760, 503),(468, 160),(524, 196),(695, 238),(377, 444),(200, 486),(758, 887),(703, 602),(756, 628),(417, 269),(712, 453),(493, 233),(955, 191),(558, 432),(594, 433),(738, 220),(622, 110),(598, 961),(361, 311),(236, 535),(349, 544),(302, 608),(475, 694),(195, 908),(586, 215),(402, 661),(413, 672),(881, 375),(666, 583),(790, 754),(359, 836),(230, 166),(172, 786),(282, 804),(904, 263),(165, 421),(462, 219),(248, 933),(525, 675),(284, 731),(901, 893),(689, 276),(736, 641),(381, 243),(185, 225),(945, 604),(225, 190),(508, 342),(110, 703),(118, 447),(734, 616),(246, 543),(397, 634),(868, 492),(544, 704),(282, 276),(139, 224),(565, 716),(265, 806),(554, 866),(765, 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522),(618, 703),(409, 943),(802, 408),(793, 318),(793, 508),(981, 279),(379, 359),(110, 774),(918, 949),(124, 885),(153, 208),(119, 608),(866, 689),(235, 774),(333, 512),(128, 577),(156, 687),(286, 387),(667, 286),(417, 328),(158, 903),(358, 476),(233, 207),(659, 819),(970, 106),(705, 897),(468, 484),(833, 241),(933, 688),(918, 202),(251, 726),(207, 384),(885, 546),(471, 631),(975, 311),(665, 152),(764, 605),(120, 735),(960, 399),(481, 345),(271, 303),(821, 522),(900, 633),(549, 360),(302, 854),(349, 584),(905, 388),(973, 479),(185, 977),(838, 341),(239, 175),(740, 163),(896, 714),(298, 440),(232, 817),(410, 178),(358, 445),(265, 283),(225, 245),(384, 538),(987, 904),(551, 506),(700, 914),(358, 397),(432, 864),(280, 991),(960, 541),(796, 598),(325, 548),(685, 567),(220, 622),(494, 379),(419, 988),(149, 411),(702, 560),(644, 444),(851, 922),(674, 773),(260, 125),(199, 620),(795, 933),(634, 825),(304, 611),(627, 697),(654, 738),(979, 785),(874, 319),(620, 567),(674, 937),(649, 839),(964, 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analyze table t;

[performance]
stats-lease = "0"

mysql> explain format='verbose' select * from tnormal use index(a) where a<100;
+-------------------------------+---------+----------+-----------+---------------------------+------------------------------------+
| id                            | estRows | estCost  | task      | access object             | operator info                      |
+-------------------------------+---------+----------+-----------+---------------------------+------------------------------------+
| IndexLookUp_7                 | 9.87    | 19260.45 | root      |                           |                                    |
| ├─IndexRangeScan_5(Build)     | 9.87    | 2008.22  | cop[tikv] | table:tnormal, index:a(a) | range:[-inf,100), keep order:false |
| └─TableRowIDScan_6(Probe)     | 9.87    | 2243.17  | cop[tikv] | table:tnormal             | keep order:false                   |
+-------------------------------+---------+----------+-----------+---------------------------+------------------------------------+
3 rows in set (0.00 sec)

mysql> explain format='verbose' select * from tnormal where a<100;
+-------------------------+---------+-----------+-----------+---------------+-------------------------+
| id                      | estRows | estCost   | task      | access object | operator info           |
+-------------------------+---------+-----------+-----------+---------------+-------------------------+
| TableReader_7           | 7.89    | 18547.23  | root      |               | data:Selection_6        |
| └─Selection_6           | 7.89    | 277207.97 | cop[tikv] |               | lt(test.tnormal.a, 100) |
|   └─TableFullScan_5     | 1000.00 | 227307.97 | cop[tikv] | table:tnormal | keep order:false        |
+-------------------------+---------+-----------+-----------+---------------+-------------------------+
3 rows in set (0.00 sec)