ganeshashree opened a new pull request, #57630:
URL: https://github.com/apache/spark/pull/57630
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### What changes were proposed in this pull request?
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This PR adds support for the ANSI SQL `UNNEST` collection derived table in
the FROM clause:
```sql
UNNEST ( expression [ , ... ] ) [ WITH ORDINALITY ] [ table_alias ]
```
UNNEST expands one or more arrays into a relation, producing one row per
element:
- Multiple arrays are expanded in parallel: the number of output rows equals
the length of the longest array, and shorter arrays are padded with NULLs.
- A NULL array is treated as an empty array and contributes no rows (a NULL
element, by contrast, produces a NULL row).
- WITH ORDINALITY appends a trailing 1-based BIGINT position column.
- Correlated arrays are supported via LATERAL (FROM t, LATERAL
UNNEST(t.arr)), reusing the existing lateral-join
machinery, and LEFT JOIN LATERAL UNNEST(...) ON true preserves outer rows
whose array is empty or NULL.
- Only ARRAY arguments are accepted. Each array contributes exactly one
output column holding its element as-is; unlike inline, an array of structs is
not expanded into one column per field.
The semantics follow PostgreSQL and Trino (parallel multi-array expansion
with NULL padding, 1-based WITH ORDINALITY). BigQuery's single-array UNNEST and
0-based WITH OFFSET are intentionally not adopted.
**References:**
- PostgreSQL:
https://www.postgresql.org/docs/17/queries-table-expressions.html
- Trino: https://trino.io/docs/current/sql/select.html
- BigQuery:
https://cloud.google.com/bigquery/docs/reference/standard-sql/query-syntax
Implementation notes:
- Grammar: UNNEST and ORDINALITY are added as non-reserved keywords (like
PIVOT/UNPIVOT), so existing identifiers named unnest/ordinality keep working. A
dedicated relationPrimary alternative (#unnestTable) and an unnest production
are added to SqlBaseParser.g4, with matching lexer tokens. A table-valued
function named unnest can still be invoked by quoting the name: `unnest`(...).
- A new Unnest generator expression implements the zip-and-pad plus
ordinality semantics. AstBuilder.visitUnnestTable desugars UNNEST into a
Generate over a OneRowRelation, reusing the shared FROM-clause aliasing helper
(mayApplyAliasPlan). UnnestTableContext is whitelisted in the two LATERAL
validation sites so correlated and LEFT JOIN LATERAL forms work.
- Unnest uses interpreted evaluation (CodegenFallback) with a lazy iterator
(one output row built per pull), consistent with other non-CollectionGenerator
generators such as ReplicateRows. A dedicated whole-stage-codegen path is left
as future work (tracked separately) and is documented in the class scaladoc.
Example:
-- multiple arrays, parallel expansion with NULL padding, plus ordinality
SELECT * FROM UNNEST(array(1, 2), array(10, 20, 30)) WITH ORDINALITY AS t(a,
b, ord);
+----+---+---+
| a| b|ord|
+----+---+---+
| 1| 10| 1|
| 2| 20| 2|
|NULL| 30| 3|
+----+---+---+
### Why are the changes needed?
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UNNEST is part of the SQL:2016 standard and is available in PostgreSQL,
Trino/Presto, and BigQuery (and, via FLATTEN, Snowflake). Spark has no
equivalent ANSI syntax today: users must rewrite UNNEST(...) to LATERAL VIEW
EXPLODE or the DataFrame explode API. This is a recurring pain point in
migrations when porting queries from other engines. Array expansion is an
everyday operation, so this closes a genuine standards-compliance gap with
broad reach.
### Does this PR introduce _any_ user-facing change?
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Yes. UNNEST(...) [WITH ORDINALITY] can now be used in the FROM clause. This
is a new feature, not a change to existing behavior; UNNEST and ORDINALITY
remain valid as regular (non-reserved) identifiers, so existing queries continue
to parse unchanged. New user-facing SQL reference documentation is added
under docs/sql-ref-syntax-qry-select-unnest.md.
### How was this patch tested?
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Added new tests:
- GeneratorExpressionSuite — unit tests for the Unnest generator: single and
multiple arrays, WITH ORDINALITY, empty/NULL arrays, column naming/nullability,
lazy evaluation (elements read only as rows are pulled), string/EXPLAIN
representation, and type-check errors.
- PlanParserSuite — parser tests: single/multiple arrays, WITH ORDINALITY,
table and column aliases, LATERAL correlation, non-reserved keyword backwards
compatibility (unnest/ordinality as identifiers), and the quoted-name TVF
escape hatch.
- SQLQueryTestSuite golden-file unnest.sql — end-to-end coverage including
parallel padding, NULL vs empty arrays, NULL elements, nested arrays, LEFT JOIN
LATERAL outer-row preservation, arrays of structs, and error cases (non-array
and MAP arguments).
Regression: existing suites that reference unnest/ordinality (e.g.
postgreSQL/with.sql, which uses ordinality as a CTE and table name) pass
without golden-file regeneration, and generators.sql,
generators-resolution-edge-cases.sql, join-lateral.sql, and postgreSQL/join.sql
all pass.
### Was this patch authored or co-authored using generative AI tooling?
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Generated-by: Claude Code (Opus 4.8)
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