[DISCUSS] organization of functions #9100

Is your feature request related to a problem or challenge?

As we break the function library out of the DataFusion core as part of #8045, we need to organize it somehow

As pointed out by @viirya on #8705 (comment) there are many potential ways to organize the data fusion function library. Thus I would like to get some consensus on how we want the organization to look before creating tickets and starting to crank it out

Describe the solution you'd like

Here is a proposal for how the functions are organized.

math functions

• feature_flag (new): math_expressions
• code location: datafusion/functions/src/math
• Abs, Acos, Asin, Atan, Atan2, Acosh, Asinh, Atanh, Cbrt, Ceil, Cos, Cosh, Degrees, Exp, Factorial, Floor, Gcd, Lcm, Ln, Log, Log10, Log2, Pi, Power, Radians, Signum, Sin, Sinh, Sqrt, Tan, Tanh, Trunc, Cot, Round, iszero
• Isnan: is the value NaN
• Nanvl: return the first non-NaN value

array functions

Given the size and specialization of these functions I propose putting them into their own subcrate

• feature_flag (new): array_expressions
• code location: datafusion/functions-array/src/math
• ArrayAppend, ArraySort, ArrayConcat, ArrayHas, ArrayHasAll, ArrayHasAny, ArrayPopFront, ArrayPopBack, ArrayDims, ArrayDistinct, ArrayElement, ArrayEmpty, ArrayLength, ArrayNdims, ArrayPosition, ArrayPositions, ArrayPrepend, ArrayRemove, ArrayRemoveN, ArrayRemoveAll, ArrayRepeat, ArrayReplace, ArrayReplaceN, ArrayReplaceAll, ArraySlice, ArrayToString, ArrayIntersect, ArrayUnion, ArrayExcept, Cardinality, ArrayResize, Flatten, Range, StringToArray,
• MakeArray: construct an array from columns (union/except depends on this)

Core functions

These functions are always available as they are used for internal purposes (like implementing [1,2,3] syntax in SQL or so commonly used that it is not worth having a feature flag)

• feature_flag: NONE
• code location: datafusion/functions/src/core or similar
• Coalesce: return the first non-null value
• Struct: Create a struct
• NullIf: return null if the two values are equal
• Random: return a random number
• ArrowTypeOf: return the arrow type of a value

Crypto functions

• feature_flag (existing): crypto_expressions
• code location: datafusion/functions/src/crypto
• Digest, MD5, SHA224, SHA256, SHA384, SHA512

String functions

• feature_flag (new): string_expressions
• code location: datafusion/functions/src/string
• ascii, bit_length, btrim, chr, concat, concat_ws, ends_with, initcap, instr, lower, ltrim, octet_length, repeat, replace, rtrim, split_part, starts_with, to_hex, trim, upper, levenshtein, uuid, overlay

Unicode string functions

These expressions need an additioanl dependency, which is why they have a different flag)

• feature_flag (existing): unicode_expressions
• code location: datafusion/functions/src/string/unicode
• CharLength, Left, Lpad, Reverse, Right, Rpad, Strpos, Substr, Translate, SubstrIndex, FindInSet

regex functions

• feature_flag (existing): regex_expressions
• code location: datafusion/functions/src/regexp
• RegexpMatch, RegexpReplace

date time function

• feature_flag (new): datetime_expressions
• code location: datafusion/functions/src/datetime
• date_part, date_trunc, date_bin, to_timestamp, to_timestamp_millis, to_timestamp_micros, to_timestamp_nanos, to_timestamp_seconds, from_unixtime, now, current_date, current_time

Describe alternatives you've considered

We can have more fine grained crates, or different organizations, etc

For example, perhaps we should pull the string functions into datafusion/functions-string crate

Additional context

No response