Identify and support push down for common functions

[zanmato1984]

In sprint 4, we plan to support:

• Mod MayPOC (support function mod in TiFlash #2318)
• COALESCE MayPOC (Push down function COALESCE #2334)
• ASCII, length, position, trim (Push down string related functions - ASCII, length, position, trim #2424)
• ADDDATE, DATE, DATE_ADD (Push down date related functions - ADDDATE, DATE, DATE_ADD #2425)
• ROUND() on Decimal (Push down ROUND() on Decimal types #2426)
• INET_NTOA()/ATON(), INET6_ATON/NTOA() (Push down INET functions - INET_NTOA()/ATON(), INET6_ATON/NTOA() #2427)
• set、enum、INET_NTOA()、INET6_NTOA()、+/- INTERVAL X Y used quite a bit【Results Generation】
• Math (Support pushdown mathematics functions #1901): Ln Log Log2 Log10 Exp Pow Radians Degrees Conv CRC32
• Comparison (Support IsTrueWithNull, IsTrue and IsFalseWithNull, IsFalse in TiFlash #1863): IsTrueWithNull, IsTrueWithoutNull, IsFalsity, NullEQ
• Bit op (Support pushdown LeftShift and RightShift bit operations  #1900 and Support pushdown LogicalXor function #1902): LeftShift, RightShift, LogicXor
• Group concat distinct (Support group concat distinct agg function push down #1908)

Sprint 3 done:

• Replace (support function replace in TiFlash #1860)
• Abs (Support abs function push down #1937)
• left/right (Support left/right function push down #1943)
• str_to_date (Support str_to_date function push down #1942)
• concat (Support concat function push down #1940)
• unix_timestamp (Support unix_timestamp function push down #1939)
• cast string as double (Support cast_str_as_double function push down #1936)
• datediff, datesub, year, day, CastStringAsInt, CastTimeAsString, concat_ws, ifnull (Need support more function pushdown in TiFlash #1480)
• Math (Support pushdown mathematics functions #1901): Ceil, Ceiling, Floor, Sqrt

List by PM: https://docs.google.com/spreadsheets/d/1FUoFEhXXVFskkBLtTfGI0XqlgveW_IdFTOgvYD7PNcA/edit#gid=508440363

[leiysky]

Here's a list of expreesions supporte by TiKV but not supported by TiFlash:

• Op functions: LeftShift, RightShift, LogicXor
• Comparison funcitons: IsTrueWithNull, IsTrueWithoutNull, IsFalsity, NullEQ
• Math functions: Ceil, Ceiling, Floor, Sqrt, Sign, Ln, Log, Log2, Log10, Exp, Pow, Radians, Degrees, Conv, CRC32
• Control functions: Coalesce
• String functions: Length, BitLength, Concat, ConcatWS, Replace, ASCII, Hex, Reverse, LTrim, RTrim, Strcmp, Space, Elt, Field
• JSON functions: JSONType, JSONExtract, JSONObject, JSONArray, JSONMerge, JSONSet, JSONInsert, JSONRemove, JSONLength
• Datetime functions: FromDays, DayOfYear, DayOfMonth, Year, Month, PeriodAdd, PeriodDiff
• Cast functions: cast any as duration, cast duration as any

I suggest we can implement all Op funcitons and Comparison functions first, since they are common and well defined.

[leiysky]

Comparison functions: #1863

[leiysky]

Enum expressions: #1880

[zanmato1984]

group_concat with distinct support is required by some random customer.

Clickhouse has groupArray and groupArrayUniq as similar function, we can leverage them. See ClickHouse/ClickHouse#13195.

Tracking in #1908

[fzhedu]

str_to_date needed from customers

[lilinghai]

Could we support the mod function push down? The CH-benCHmark contains a lot of queries that contains mod function.
https://db.in.tum.de/research/projects/CHbenCHmark/?lang=en

[leiysky]

From PingCAP Metabase:

• Lower
• Date