add tikv lower/upper functions (#15860)

functions-and-operators/expressions-pushed-down.md

@@ -21,7 +21,7 @@ TiFlash also supports pushdown for the functions and operators [listed on this p
 | [Control flow functions](/functions-and-operators/control-flow-functions.md) | [CASE](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#operator_case), [IF()](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_if), [IFNULL()](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_ifnull) |
 | [JSON functions](/functions-and-operators/json-functions.md) | [JSON_ARRAY([val[, val] ...])](https://dev.mysql.com/doc/refman/8.0/en/json-creation-functions.html#function_json-array),<br/> [JSON_CONTAINS(target, candidate[, path])](https://dev.mysql.com/doc/refman/8.0/en/json-search-functions.html#function_json-contains),<br/> [JSON_EXTRACT(json_doc, path[, path] ...)](https://dev.mysql.com/doc/refman/8.0/en/json-search-functions.html#function_json-extract),<br/> [JSON_INSERT(json_doc, path, val[, path, val] ...)](https://dev.mysql.com/doc/refman/8.0/en/json-modification-functions.html#function_json-insert),<br/> [JSON_LENGTH(json_doc[, path])](https://dev.mysql.com/doc/refman/8.0/en/json-attribute-functions.html#function_json-length),<br/> [JSON_MERGE(json_doc, json_doc[, json_doc] ...)](https://dev.mysql.com/doc/refman/8.0/en/json-modification-functions.html#function_json-merge),<br/> [JSON_OBJECT([key, val[, key, val] ...])](https://dev.mysql.com/doc/refman/8.0/en/json-creation-functions.html#function_json-object),<br/> [JSON_REMOVE(json_doc, path[, path] ...)](https://dev.mysql.com/doc/refman/8.0/en/json-modification-functions.html#function_json-remove),<br/> [JSON_REPLACE(json_doc, path, val[, path, val] ...)](https://dev.mysql.com/doc/refman/8.0/en/json-modification-functions.html#function_json-replace),<br/> [JSON_SET(json_doc, path, val[, path, val] ...)](https://dev.mysql.com/doc/refman/8.0/en/json-modification-functions.html#function_json-set),<br/> [JSON_TYPE(json_val)](https://dev.mysql.com/doc/refman/8.0/en/json-attribute-functions.html#function_json-type),<br/> [JSON_UNQUOTE(json_val)](https://dev.mysql.com/doc/refman/8.0/en/json-modification-functions.html#function_json-unquote),<br/> [JSON_VALID(val)](https://dev.mysql.com/doc/refman/8.0/en/json-attribute-functions.html#function_json-valid),<br/> [value MEMBER OF(json_array)](https://dev.mysql.com/doc/refman/8.0/en/json-search-functions.html#operator_member-of) |
 | [Date and time functions](/functions-and-operators/date-and-time-functions.md) | [DATE()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_date), [DATE_FORMAT()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_date-format), [DATEDIFF()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_datediff), [DAYOFMONTH()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_dayofmonth), [DAYOFWEEK()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_dayofweek), [DAYOFYEAR()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_dayofyear), [FROM_DAYS()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_from-days), [HOUR()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_hour), [MAKEDATE()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_makedate), [MAKETIME()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_maketime), [MICROSECOND()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_microsecond), [MINUTE()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_minute), [MONTH()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_month), [MONTHNAME()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_monthname), [PERIOD_ADD()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_period-add), [PERIOD_DIFF()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_period-diff), [SEC_TO_TIME()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_sec-to-time), [SECOND()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_second), [SYSDATE()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_sysdate), [TIME_TO_SEC()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_time-to-sec), [TIMEDIFF()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_timediff), [WEEK()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_week), [WEEKOFYEAR()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_weekofyear), [YEAR()](https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_year) |
-| [String functions](/functions-and-operators/string-functions.md) | [ASCII()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_ascii), [BIT_LENGTH()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_bit-length), [CHAR()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_char), [CHAR_LENGTH()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_char-length), [CONCAT()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_concat), [CONCAT_WS()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_concat-ws), [ELT()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_elt), [FIELD()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_field), [HEX()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_hex), [LENGTH()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_length), [LIKE](https://dev.mysql.com/doc/refman/8.0/en/string-comparison-functions.html#operator_like), [LTRIM()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_ltrim), [MID()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_mid), [NOT LIKE](https://dev.mysql.com/doc/refman/8.0/en/string-comparison-functions.html#operator_not-like), [NOT REGEXP](https://dev.mysql.com/doc/refman/8.0/en/regexp.html#operator_not-regexp), [REGEXP](https://dev.mysql.com/doc/refman/8.0/en/regexp.html#operator_regexp), [REPLACE()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_replace), [REVERSE()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_reverse), [RIGHT()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_right), [RTRIM()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_rtrim), [SPACE()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_space), [STRCMP()](https://dev.mysql.com/doc/refman/8.0/en/string-comparison-functions.html#function_strcmp), [SUBSTR()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_substr), [SUBSTRING()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_substring) |
+| [String functions](/functions-and-operators/string-functions.md) | [ASCII()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_ascii), [BIT_LENGTH()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_bit-length), [CHAR()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_char), [CHAR_LENGTH()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_char-length), [CONCAT()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_concat), [CONCAT_WS()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_concat-ws), [ELT()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_elt), [FIELD()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_field), [HEX()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_hex), [LENGTH()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_length), [LIKE](https://dev.mysql.com/doc/refman/8.0/en/string-comparison-functions.html#operator_like), [LOWER()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_lower), [LTRIM()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_ltrim), [MID()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_mid), [NOT LIKE](https://dev.mysql.com/doc/refman/8.0/en/string-comparison-functions.html#operator_not-like), [NOT REGEXP](https://dev.mysql.com/doc/refman/8.0/en/regexp.html#operator_not-regexp), [REGEXP](https://dev.mysql.com/doc/refman/8.0/en/regexp.html#operator_regexp), [REPLACE()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_replace), [REVERSE()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_reverse), [RIGHT()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_right), [RTRIM()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_rtrim), [SPACE()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_space), [STRCMP()](https://dev.mysql.com/doc/refman/8.0/en/string-comparison-functions.html#function_strcmp), [SUBSTR()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_substr), [SUBSTRING()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_substring), [UPPER()](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_upper) |
 | [Aggregation functions](/functions-and-operators/aggregate-group-by-functions.md#aggregate-group-by-functions) | [COUNT()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_count), [COUNT(DISTINCT)](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_count-distinct), [SUM()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_sum), [AVG()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_avg), [MAX()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_max), [MIN()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_min), [VARIANCE()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_variance), [VAR_POP()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_var-pop), [STD()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_std), [STDDEV()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_stddev), [STDDEV_POP](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_stddev-pop), [VAR_SAMP()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_var-samp), [STDDEV_SAMP()](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_stddev-samp), [JSON_ARRAYAGG(key)](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_json-arrayagg), [JSON_OBJECTAGG(key, value)](https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_json-objectagg) |
 | [Encryption and compression functions](/functions-and-operators/encryption-and-compression-functions.md#encryption-and-compression-functions) | [MD5()](https://dev.mysql.com/doc/refman/8.0/en/encryption-functions.html#function_md5), [SHA1(), SHA()](https://dev.mysql.com/doc/refman/8.0/en/encryption-functions.html#function_sha1), [UNCOMPRESSED_LENGTH()](https://dev.mysql.com/doc/refman/8.0/en/encryption-functions.html#function_uncompressed-length) |
 | [Cast functions and operators](/functions-and-operators/cast-functions-and-operators.md#cast-functions-and-operators) | [CAST()](https://dev.mysql.com/doc/refman/8.0/en/cast-functions.html#function_cast), [CONVERT()](https://dev.mysql.com/doc/refman/8.0/en/cast-functions.html#function_convert) |