consider adding remainder of postgres functions to supported functions

[uladkasach]

so far, only a subset are supported from:

• https://www.postgresql.org/docs/10/functions-math.html
• https://www.postgresql.org/docs/10/functions-string.html

in the mean time, users can always just wrap queries that need an unsupported function in a function and call that from the query, so this is not a blocker