Support mathematical functions: conv, crc32, mod, pow/power, round, sign, sqrt, truncate

[chloe-zh]

Issue #, if available:

Description of changes:

• conv
  usage: conv(x, a, b) converts the number x from a base to b base
  argument type: x: string, a: integer, b: integer
  return type: string

• crc32
  usage: calculates a cyclic redundancy check value and returns a 32-bit unsigned value
  argument type: string
  return type: long

• mod
  usage: mod(n, m) calculates the remainder of the number n divided by m
  argument type: integer/long/float/double
  return type: widen type between types of n and m

• pow/power
  usage: pow(x, y) calculates the value of x raised to the power of y
  argument type: integer/long/float/double
  return type: double

• round
  usage: round(x, d) rounds the argument x to d decimal places, d defaults to 0 if not specified
  argument type: integer/long/float/double
  return type map:
  (integer/long [,integer]) -> long
  (float/double [,integer]) -> double

• sign
  usage: returns the sign of the argument as -1, 0, or 1, depending on whether the number is negative, zero, or positive
  argument type: integer/long/float/double
  return type: integer

• sqrt
  usage: calculate the square root of a nonnegative number
  argument type: integer/long/float/double
  return type map:
  (nonnegative) integer/long/float/double -> double
  (negative) integer/long/float/double -> null

• truncate
  usage: truncate(x, d) returns the number x, truncated to d decimal places
  argument type: integer/long/float/double
  return type map:
  integer/long -> long
  float/double -> double

Reference

• MySQL Reference Munual
• ArterixDB Numeric Functions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[dai-chen]

Could you check if we can avoid and fix this issue in new engine? #331

[dai-chen]

I think the description of this PR should be added to our doc. Could you add it and make this a doctest file: https://github.com/opendistro-for-elasticsearch/sql/blob/master/docs/user/dql/functions.rst

[chloe-zh]

Could you check if we can avoid and fix this issue in new engine? #331

MySQL returns null for functions with illegal arguments, like ln(-1), log(-1) etc, and for the result of infinity, and we keep consistent with this behavior. But for special cases like pow(0, 0) (Java behavior is returning 1), I didn't find any reference in MySQL manual, so for now it still follows the java behaviors.

[dai-chen]

Could you check if we can avoid and fix this issue in new engine? #331

MySQL returns null for functions with illegal arguments, like ln(-1), log(-1) etc, and for the result of infinity, and we keep consistent with this behavior. But for special cases like pow(0, 0) (Java behavior is returning 1), I didn't find any reference in MySQL manual, so for now it still follows the java behaviors.

Sounds good! I just tested pow(0,0) in MySQL. It returns 1 same as Java. :)

[dai-chen]

LGTM. Thanks for the changes!