--- title: "unnest" slug: "dev-database-reference-sql-functions-table-functions-unnest" updated: 2026-10-30T18:18:46Z published: 2026-10-30T18:18:46Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-table-functions-unnest" --- > ## Documentation Index > Fetch the complete documentation index at: https://docs.vastdata.com/llms.txt > Use this file to discover all available pages before exploring further. # unnest *Applies to VAST Cluster 5.5.* *[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [Table functions](/develop/docs/dev-database-reference-sql-functions-table-functions) › Rows from a list.* Turns a list into rows, one row per element. In `FROM`, it expands a list you give it; in the `SELECT` list of a query on a table, it expands a list column, repeating the row's other columns for each element. ## Syntax ``` unnest(list) unnest(list, recursive := true) unnest(struct) ``` | Argument | Type | Description | | --- | --- | --- | | `list` | A list of any element type (`ANY[]`) | The list to expand: a literal or expression in `FROM`, or a list column in the `SELECT` list. | | `recursive` | `BOOLEAN` | In the `SELECT` list only: `true` expands nested lists all the way down. | | `struct` | A struct | In the `SELECT` list only: turns the struct's fields into columns. | **Returns:** one row per element, of the element type. In `FROM`, the column is named `unnest`; give it an alias: `FROM unnest([1, 2]) t(v)`. ## Set up the examples The examples connect with the ADBC driver, as in [VAST Query Engine (ADBC)](/develop/docs/dev-database-integrations-adbc), and work in a schema called `docs_sql_fn_table_functions_unnest`. Set `QE_ENDPOINT`, `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY` and `VASTDB_BUCKET` ([SQL reference](/develop/docs/dev-database-reference-sql-index#example-connect-and-use-a-transaction)), and run the blocks in order in one Python session. Below each block is the output you should see. This block connects, defines the `run()` and `plan()` helpers, and creates a table of four articles with a list of tags. Article 2 has no tags, article 3 has a NULL list, and one of article 4's tags is NULL: ``` import os import re import adbc_driver_manager import adbc_driver_manager.dbapi as dbapi import adbc_driver_vastdb BUCKET = os.environ["VASTDB_BUCKET"] SCHEMA = f'"{BUCKET}/docs_sql_fn_table_functions_unnest"' conn = dbapi.connect( driver=adbc_driver_vastdb.get_driver_path(), db_kwargs={ "vast.db.endpoint": os.environ["QE_ENDPOINT"], "vast.db.access_key": os.environ["AWS_ACCESS_KEY_ID"], "vast.db.secret_key": os.environ["AWS_SECRET_ACCESS_KEY"], }, autocommit=True, ) def server_message(e): """The server's message in a driver error, without the request ID or the echoed statement.""" text = re.sub(r"\[req=0x[0-9a-f]+\]: ", "", str(e)) found = (re.search(r'message: "(.*?)", request_id', text, re.S) or re.search(r"description: (.*)\)", text, re.S)) message = found.group(1) if found else text.strip().splitlines()[-1] message = re.sub(r"^Plan query: '.*?' failed with: ", "", message, flags=re.S) message = re.split(r"\\n|\n", message)[0].replace('\\"', '"') return message.replace(BUCKET, "") def run(sql): """Run one statement. Print the rows it returns, or the error.""" try: with conn.cursor() as cur: cur.execute(sql) rows = cur.fetchall() if cur.description else [] except (adbc_driver_manager.Error, OSError) as e: print(f"{type(e).__name__}: {server_message(e)}") else: print(rows) def plan(sql): """Print the plan without its ~N Rows lines: the table's row count, which can still be 0 just after an insert.""" with conn.cursor() as cur: cur.execute("EXPLAIN " + sql) steps = cur.fetchall()[0][1] print("\n".join(line for line in steps.splitlines() if "Rows" not in line).replace(BUCKET, "")) TABLE = f"{SCHEMA}.articles" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, title VARCHAR, tags VARCHAR[])") run(f"INSERT INTO {TABLE} VALUES " "(1, 'Intro', ['sql', 'howto']), " "(2, 'Notes', []), " "(3, 'Draft', NULL), " "(4, 'Guide', ['vector', NULL, 'sql'])") ``` ``` [] [] [(4,)] ``` ## What it does `unnest` works in two places: | Where | What it expands | Example | | --- | --- | --- | | `FROM` | A list literal or an expression that returns a list | `SELECT * FROM unnest(['x', 'y'])` returns rows `'x'` and `'y'` | | The `SELECT` list of a query on a table | A list column, one table row at a time | `SELECT id, unnest(tags) FROM articles` returns one row per tag, with the article's `id` | A list of lists is expanded one level: `unnest([[1, 2], [3]])` gives the rows `[1, 2]` and `[3]`. A list of structs gives one struct per row. In the `SELECT` list, `unnest(list, recursive := true)` expands every level, so `[[1, 2], [3]]` gives `1`, `2` and `3`, and `unnest(struct)` turns a struct into one column per field. Two `unnest()` calls in one `SELECT` list are expanded side by side: the first elements form the first row, and so on. The shorter list is padded with NULLs. ## NULLs and edge cases | Input | Result | | --- | --- | | An empty list, or a typed NULL list such as `NULL::INTEGER[]` | No rows. In the `SELECT` list, the table row doesn't appear in the result. | | A NULL element, such as `[1, NULL, 3]` | A NULL row for that element | | An untyped `NULL`, a number, a string, a struct or a map in `FROM` | Error: `Binder Error: UNNEST requires a single list as input` | | A fixed-size array, such as `[1, 2, 3]::INTEGER[3]` | Error: `Binder Error: UNNEST requires a single list as input`; cast it to a list, `::INTEGER[]` | | Two lists in `FROM`, such as `unnest([1, 2], [10, 20, 30])` | Error: `No function matches the given name and argument types 'unnest(INTEGER[], INTEGER[])'` | | `recursive := true` in `FROM` | Error: `Binder Error: Invalid named parameter "recursive" for function unnest` | The same cases, as the cluster returns them. They use the helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-table-functions-unnest#set-up-the-examples): ``` run("SELECT * FROM unnest([]::INTEGER[])") run("SELECT * FROM unnest(NULL::INTEGER[])") run("SELECT * FROM unnest([1, NULL, 3])") run("SELECT * FROM unnest([[1, 2], [3]])") run("SELECT * FROM unnest(NULL)") run("SELECT * FROM unnest([1, 2, 3]::INTEGER[3])") run("SELECT * FROM unnest([1, 2], [10, 20, 30])") run("SELECT * FROM unnest([[1, 2], [3]], recursive := true)") ``` ``` [] [] [(1,), (None,), (3,)] [([1, 2],), ([3],)] InternalError: Binder Error: UNNEST requires a single list as input InternalError: Binder Error: UNNEST requires a single list as input InternalError: Binder Error: No function matches the given name and argument types 'unnest(INTEGER[], INTEGER[])'. You might need to add explicit type casts. InternalError: Binder Error: Invalid named parameter "recursive" for function unnest ``` ## Examples Rows from a list built by another function: ``` run("SELECT * FROM unnest(string_split('a,b,c', ',')) t(part)") run("SELECT * FROM unnest(range(3))") ``` ``` [('a',), ('b',), ('c',)] [(0,), (1,), (2,)] ``` One row per tag of each article. Articles 2 and 3 have no tags, so they don't appear; article 4's NULL tag gives a NULL row. `ORDER BY` on the article alone doesn't keep the tags of an article in list order, so sort by the tag too: ``` run(f"SELECT id, unnest(tags) AS tag FROM {TABLE} ORDER BY id, tag LIMIT 20") ``` ``` [(1, 'howto'), (1, 'sql'), (4, 'sql'), (4, 'vector'), (4, None)] ``` The tags of one article, and the number of tags of all articles: ``` run(f"SELECT id, title, unnest(tags) AS tag FROM {TABLE} WHERE id = 1") run(f"SELECT count(*) FROM (SELECT unnest(tags) FROM {TABLE})") ``` ``` [(1, 'Intro', 'sql'), (1, 'Intro', 'howto')] [(5,)] ``` Two lists side by side, a nested list expanded all the way down, and a struct turned into columns: ``` run("SELECT unnest([1, 2]), unnest(['a', 'b', 'c'])") run("SELECT unnest([[1, 2], [3]], recursive := true)") run("SELECT unnest({'a': 1, 'b': 'x'})") ``` ``` [(1, 'a'), (2, 'b'), (None, 'c')] [(1,), (2,), (3,)] [(1, 'x')] ``` ## How it runs in VAST The Query Engine runs on the cluster's CNodes, and a query on a table starts with a scan in VAST DataBase (`VAST_SCAN`). - **In `FROM`,** `unnest` replaces the scan: it produces its rows in the Query Engine, and the plan shows an `UNNEST` step where `VAST_SCAN` would be. A `WHERE` condition on the rows runs in a `FILTER` step above it. - **In the `SELECT` list,** the scan reads the list column in VAST DataBase, and an `UNNEST` step above it turns each list into rows. A `WHERE` condition on the table's columns, such as `id = 1`, is evaluated in the scan, before the lists are expanded. The plans show both: ``` plan("SELECT * FROM unnest([1, 2, 3]) t(v) WHERE v > 1") plan(f"SELECT id, unnest(tags) AS tag FROM {TABLE} WHERE id = 1") ``` ``` ┌───────────────────────────┐ │ FILTER │ │ ──────────────────── │ │ (v > 1) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ UNNEST │ │ ──────────────────── │ │ Function: UNNEST │ │ │ └───────────────────────────┘ ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ id │ │ tag │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ UNNEST │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ id │ │ tags │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Expand list columns in the `SELECT` list.** `SELECT id, unnest(tags) FROM t` reads only `id` and `tags`, and conditions on the other columns still push down to VAST DataBase ([pushdowns](/develop/docs/dev-database-reference-pushdowns)). - **Filter rows before you expand them.** Put conditions on the table's columns in `WHERE` of the same query, as in `WHERE id = 1` above, so fewer lists are expanded. ## Limitations and errors - **Not in `WHERE`.** `WHERE unnest(tags) = 'sql'` fails with `Binder Error: UNNEST not supported here`. To keep the rows that have a value in a list, use [list_contains](/develop/docs/dev-database-reference-sql-functions-lists-list-contains). - **No lateral `unnest` in `FROM`.** `FROM t, unnest(t.tags)` and `FROM t CROSS JOIN unnest(tags)` fail with `The operator 'HASH_GROUP_BY' type=9 is currently not supported.` Use `unnest()` in the `SELECT` list. - **No nested calls.** `unnest(unnest(x))` fails with `Binder Error: Nested UNNEST calls are not supported - use UNNEST(x, recursive := true) to unnest multiple levels`. - **No `GROUP BY` on the elements.** It fails with `The operator 'HASH_GROUP_BY' type=9 is currently not supported.` An aggregate over all the elements, such as `count(*)`, works. - **`ORDER BY` needs a `LIMIT`.** Without one it fails with `The operator 'ORDER_BY' type=1 is currently not supported for this usage.` ``` run(f"SELECT id FROM {TABLE} WHERE unnest(tags) = 'sql'") run(f"SELECT id, tag FROM {TABLE}, unnest(tags) t(tag)") run("SELECT unnest(unnest([[1, 2], [3]]))") run(f"SELECT tag, count(*) FROM (SELECT unnest(tags) AS tag FROM {TABLE}) GROUP BY tag ORDER BY tag LIMIT 20") run(f"SELECT id FROM {TABLE} WHERE list_contains(tags, 'sql') ORDER BY id LIMIT 10") ``` ``` InternalError: Binder Error: UNNEST not supported here InternalError: The operator 'HASH_GROUP_BY' type=9 is currently not supported. InternalError: Binder Error: Nested UNNEST calls are not supported - use UNNEST(x, recursive := true) to unnest multiple levels InternalError: The operator 'HASH_GROUP_BY' type=9 is currently not supported. [(1,), (4,)] ``` ## Similar functions | Function | Turns | Into | | --- | --- | --- | | `unnest` | a list | one row per element | | [range](/develop/docs/dev-database-reference-sql-functions-table-functions-range) and [generate_series](/develop/docs/dev-database-reference-sql-functions-table-functions-generate-series) in `FROM` | a start, a stop and a step | one row per value | | [repeat](/develop/docs/dev-database-reference-sql-functions-table-functions-repeat) in `FROM` | a value and a count | the same value in that many rows | | [string_split](/develop/docs/dev-database-reference-sql-functions-string-string-split) | a string | a list, which `unnest` can expand | ``` run("SELECT * FROM unnest([10, 20, 30])") run("SELECT * FROM range(10, 31, 10)") run("SELECT * FROM repeat(10, 3)") ``` ``` [(10,), (20,), (30,)] [(10,), (20,), (30,)] [(10,), (10,), (10,)] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] ``` ## See also - [Table functions](/develop/docs/dev-database-reference-sql-functions-table-functions): all the table functions. - [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs): working with lists before or after `unnest()`. - [Where filters run](/develop/docs/dev-database-reference-sql-select#where-filters-run) and [pushdowns](/develop/docs/dev-database-reference-pushdowns). - [EXPLAIN](/develop/docs/dev-database-reference-sql-explain): how to read a plan.