[Go] TimestampType zero value default to seconds unit instead of nanoseconds

[ddl-gabrielhaim]

Bug

The godoc mentions the zero value of arrow.TimestampType should be nanosecond UTC (see https://github.com/apache/arrow/blob/main/go/arrow/datatype_fixedwidth.go#L340) but the arrow.TimeUnit seems to default to seconds (https://github.com/apache/arrow/blob/main/go/arrow/datatype_fixedwidth.go#L327)

package main

import (
	"fmt"

	"github.com/apache/arrow/go/v13/arrow"
)

func main() {
	t1 := arrow.TimestampType{Unit: arrow.Nanosecond}
	t2 := arrow.TimestampType{}

	fmt.Printf("Timestamp Unit 3: %s\n", t1.Unit)
	fmt.Printf("Timestamp Unit default: %s\n", t2.Unit)
}

output:

Timestamp Unit 3: ns
Timestamp Unit default: s

Component(s)

Go, Arrow v13

[zeroshade]

@ddl-gabrielhaim good catch. A bunch of the math and shortcuts are based on the fact that the enum value of Seconds is 0 (to make life easier), so we should probably update the documentation/comment to clarify that the zero value is Seconds not Nanoseconds. Would you be willing to create a PR for it?