query gets 'encoding failed' using distinct and elt function

[wjhuang2016]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE `tl4fd484a9` (
  `col_8` double NOT NULL DEFAULT '8101.804333035654',
  `col_9` bigint(20) unsigned DEFAULT NULL,
  `col_10` bit(8) DEFAULT NULL,
  `col_11` bit(42) DEFAULT NULL,
  UNIQUE KEY `idx_4` (`col_9`),
  UNIQUE KEY `idx_5` (`col_9`),
  UNIQUE KEY `idx_6` (`col_9`,`col_10`,`col_8`),
  UNIQUE KEY `idx_7` (`col_9`,`col_8`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
PARTITION BY HASH (`col_9`) PARTITIONS 2;

INSERT INTO `tl4fd484a9` VALUES (23.007995724948362,9223372036854775808,x'd5',x'036bda736b9f'), (2591.3122474206766,5752943349156553245,NULL,x'02b77969361c'), (4479.317291827713,15055294211639017111,x'aa',x'01eee6f9a4a3'), (7628.938897170623,7496567757002745451,x'eb',x'0287ad3ecd13'), (8347.197576995819,6203659976161826151,x'18',x'01d6475a8df2'), (8710.54054998306,8514535559911610148,x'e2',x'03f899c130b2'), (1195.077740002609,3436186226464611422,x'50',x'0093ab260f48'), (1013.7500694146422,7471092280998844033,x'dd',x'03939af0ec85'), (5521.611017775993,16394145724846312682,x'61',x'01c06f356d0a'), (8587.461307265945,1885935207250807462,x'60',x'035a03c4dff2'), (7650.576352691503,14483491971408231742,x'91',x'02c6eaea03a1'), (2851.908224583381,15373714602380758326,NULL,x'00b8a992625f'), (5204.233883775764,12245754788232558702,x'8c',x'005b42a9761e'), (711.5713246022093,9223372036854775807,x'15',x'03ce7c19cc28'), (6700.8257413585525,11960899368204480686,x'5a',x'01a07ebf627f'), (3715.8911161876927,16717915239921191114,x'19',x'0183db6da03f'), (5588.550656466107,3324080768693532255,x'69',x'029bfa03f27e'), (6736.377121180751,4102219363823857377,x'34',x'02389972a4bc'), (5010.654821363877,1,x'01',x'012d8efdb99d'), (2555.2202292511456,NULL,x'27',x'02a09857f11c'), (6513.333951893525,7297996079516535344,x'ff',x'00123dbb5766'), (7211.133912898516,16059815461817714863,x'88',x'000000000000'), (3223.450063930532,10556257816865671061,x'76',x'035575eb3736'), (5934.213444452291,3506157312092400137,x'2f',x'0099bb76950a'), (5613.036187642952,13165727662517512195,x'01',x'01ffb1103861'), (161.42937173636415,NULL,x'3f',x'032eedd972eb'), (8632.659024782468,16757755969895857419,x'5d',NULL), (7047.649466854864,14821390344080511959,x'a6',x'0012a171b473'), (7442.306955199656,12094326225722438418,x'12',x'000000000000'), (1795.1199576186712,1328447598647750078,x'2d',x'0325b495eba1'), (7467.011217133958,9561095661261955906,x'01',x'011834544fb1'), (2972.7118048537704,18446744073709551615,x'b1',x'01977242631c'), (3161.6599685270917,7911534160713813860,x'37',x'03782feb0295'), (9546.629394674586,17028252544791959939,x'ff',x'01d73fa1f0fa'), (2306.9705216860984,1630137914052885313,x'31',x'005d59426708'), (7601.530447792593,6986807871336145185,x'd5',x'016998085258'), (7058.842877388801,8570807740729504101,x'a5',x'0112cc74569e'), (615.6011553350702,11897798906906134285,x'34',x'03d135a573c2'), (3801.6115898094317,11678651343133193774,x'3a',x'009f3c65f2c5'), (6779.239615471537,2484649041714012147,x'65',x'03b9cba6820d');

SELECT DISTINCT ELT(2, `tl4fd484a9`.`col_8`, `tl4fd484a9`.`col_10`) AS `r0` FROM `tl4fd484a9`;

2. What did you expect to see? (Required)

No error

3. What did you see instead (Required)

mysql> SELECT DISTINCT ELT(2, `tl4fd484a9`.`col_8`, `tl4fd484a9`.`col_10`) AS `r0` FROM `tl4fd484a9`;
ERROR 1105 (HY000): encoding failed

4. What is your TiDB version? (Required)

4e41699

[windtalker]

The root cause seems TiDB think the result string of ELT(2, `tl4fd484a9`.`col_8`, `tl4fd484a9`.`col_10`) has unicode0900AICI collator, but tl4fd484a9`.`col_10 is actually a bit column, and cast(test.tl4fd484a9.col_10, var_string(1)) actually can not make the result a valid utf string if the input string is not utf valid string, so it may meet encode error. I would change the label to sig infra