Spark, Databricks: read multi-units interval literals

[moshap-firebolt]

Spark's multi-units interval syntax lists several value / unit pairs, INTERVAL 10 YEAR 20 MONTH, and documents it as the same thing as the string form INTERVAL '10 YEAR 20 MONTH'. Both dialects failed on the second pair (Expected: end of statement, found: 20).

A new dialect hook, supports_interval_multi_units (on for SparkSqlDialect and DatabricksDialect), lets parse_interval keep reading value / unit pairs after the first and return them as that string form, so the AST is unchanged. Values may be numbers (with a + or - sign) or strings. The extra pairs are read speculatively (maybe_parse), so a single unit, or a unit followed by anything else (INTERVAL 3 DAY + 1, INTERVAL 1 DAY, 2), parses as before.

Tests in tests/sqlparser_spark.rs cover two and three pairs, negative and plus-signed values, a string value, an alias after the literal, and the forms that must stay unchanged. The fork's full suite, fmt and clippy are clean.

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Note

Low Risk
Dialect-scoped SQL parsing change with speculative lookahead; no runtime execution or data-path impact beyond accepting additional interval syntax.

Overview
Adds dialect-gated support for Spark/Databricks multi-unit interval literals like INTERVAL 10 YEAR 20 MONTH, which previously failed after the first unit pair.

A new supports_interval_multi_units hook (enabled on Spark and Databricks) extends parse_interval to optionally consume additional value/unit pairs via speculative parsing and normalize them to the equivalent string literal (INTERVAL '10 YEAR 20 MONTH'), leaving the interval AST shape unchanged. Values can be numbers or quoted strings, with optional +/- signs. Single-unit forms and intervals followed by unrelated tokens (e.g. INTERVAL 3 DAY + 1) still parse as before.

Common parser tests assert round-trip normalization for supported dialects and parse failures when the hook is off.

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