[RFC] allow now like function do not use function properties to fix current timestamp.

[xinyual]

Overview
Current, we use function properties as the parameter to fix current timestamp in now like functions (now(), current_time(), current_timestamp()). This allow us to show the same current time in one query execution. For example, we call source=XX | eval a=utc_time() | fields a, it will show

{
    "schema": [
        {
            "name": "a",
            "type": "time"
        }
    ],
    "datarows": [
        [
            "07:35:29.811442"
        ],
        [
            "07:35:29.811442"
        ],
....
}

The times are same even in microsecond level.
But other engine like calcite, spark has different performance. The result depends on the time the exact line is executed. Besides, avoiding passing function properties could make the logical plan clear and allow us the initialize static function object, improving the performance. After removing the function properties in argument, the return could be something like

{
    "schema": [
        {
            "name": "a",
            "type": "time"
        }
    ],
    "datarows": [
        [
            "07:35:29.811442"
        ],
        [
            "07:35:29.835123"
        ],
        [
            "07:35:29.849123"
        ],
....
}

[LantaoJin]

Want to know the background for current implementation about function properties @penghuo .

In v3, we'd better to host a static map for all builtin functions (Spark, Calcite have the similar implementation), based on this idea, we cannot pass a function properties to the static function object any more, unless we create the function instance per query.

[penghuo]

@xinyual

The result depends on the time the exact line is executed

Does it means current_timestamp() return different result when evaluate each row? Did u test in Spark also. from doc, "current_timestamp() - Returns the current timestamp at the start of query evaluation. All calls of current_timestamp within the same query return the same value"
Simlilar in PostgreSQL, functions all return values based on the start time of the current transaction:

@LantaoJin

Want to know the background for current implementation about function properties @penghuo .

The idea is to NOW/CURRENT_TIMESTAMP return same value in same query. #754

[LantaoJin]

Checked in Spark:

current_timestamp(): Returns the current timestamp at the start of query evaluation. All calls of current_timestamp within the same query return the same value.

@xinyual I think we still need to figure out a approach for these datetime functions. Spark marks the current_timestamp as FoldableUnevaluable which means it cannot be evaluated but is guaranteed to be replaced with a foldable value by query optimizer.

[LantaoJin]

I think we can store the current time (System.currentTimeMillis()) in CalcitePlanContext.

[xinyual]

Yes. Will follow the same thing as other engine to return same value in same query

I think we can store the current time (System.currentTimeMillis()) in CalcitePlanContext.