Support INTERVAL type

[apstndb]

INTERVAL type is officially released.

https://cloud.google.com/spanner/docs/reference/standard-sql/lexical#interval_literals
https://cloud.google.com/spanner/docs/reference/standard-sql/data-types#interval_type

Current Behavior

spanner> SELECT INTERVAL 1 DAY AS `interval`;
+--------------------+
| interval           |
| UNKNOWN            |
+--------------------+
| string_value:"P1D" |
+--------------------+
1 rows in set (3.28 msecs)

Format

I recommend to use ISO 8601 duration format because:
It is used as internal representation of INTERVAL.

https://github.com/googleapis/googleapis/blob/1270a1ba7ae32ce486b28d00ced9575c29d9ff75/google/spanner/v1/type.proto#L172-L177

// Encoded as `string`, in `ISO8601` duration format -
// `P[n]Y[n]M[n]DT[n]H[n]M[n[.fraction]]S`
// where `n` is an integer.
// For example, `P1Y2M3DT4H5M6.5S` represents time duration of 1 year, 2
// months, 3 days, 4 hours, 5 minutes, and 6.5 seconds.
INTERVAL = 16;

Spanner Studio and gcloud spanner databases execute-sql uses the same format.

Implementation

$ gcloud spanner databases execute-sql ${SPANNER_DATABASE_ID} --sql 'SELECT INTERVAL 1 DAY AS `interval`'
interval
P1D

I think it is good enough to use Interval.String() and NullInterval.String() because it format value as ISO 8601 duration.
https://pkg.go.dev/cloud.google.com/go/spanner#Interval.String

String returns the ISO8601 duration format string representation of the interval.

[yfuruyama]

Thank you for filing this.

Yes, I agree that showing the Interval type with ISO8601 format is the best approach.