--- title: "union_extract" slug: "dev-database-reference-sql-functions-utility-union-extract" updated: 2026-10-30T18:18:46Z published: 2026-10-30T18:18:46Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-utility-union-extract" --- > ## 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. # union_extract *Applies to VAST Cluster 5.5.* *[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs) › Union values.* Returns the value of one member of a `UNION` value, by its tag: `union_extract(union_value(k := 1), 'k')` is `1`. If the union holds a different member, the result is `NULL`. Union values are built in a query with [union_value](/develop/docs/dev-database-reference-sql-functions-utility-union-value); a table can't have a `UNION` column. ## Syntax ``` union_extract(u, tag) ``` | Argument | Type | Description | | --- | --- | --- | | `u` | A `UNION` value | The union to read. | | `tag` | `VARCHAR` | The name of the member to return. It must be one of the union's members. | **Returns:** the type of that member: for `UNION(k INTEGER, s VARCHAR)`, `'k'` gives an `INTEGER` and `'s'` a `VARCHAR`. ## 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_utility_union_extract`. 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 five sensor readings. A reading is either a number or a text status; the last row has no values: ``` 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_utility_union_extract"' 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)) TABLE = f"{SCHEMA}.readings" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, num BIGINT, status VARCHAR)") run(f"INSERT INTO {TABLE} VALUES " "(1, 21, NULL), " "(2, NULL, 'offline'), " "(3, -4, NULL), " "(4, NULL, 'calibrating'), " "(5, NULL, NULL)") ``` ``` [] [] [(5,)] ``` ## What it does `union_extract(u, tag)` returns the member `tag` of `u` if that's the member `u` holds, and `NULL` if `u` holds another member: | Call | Result | | --- | --- | | `union_extract(union_value(k := 1), 'k')` | `1` | | `union_extract(union_value(k := 1)::UNION(k INTEGER, s VARCHAR), 'k')` | `1` | | `union_extract(union_value(k := 1)::UNION(k INTEGER, s VARCHAR), 's')` | `NULL`, a `VARCHAR` | To find out which member a union holds, use `union_tag(u)::VARCHAR`. ## NULLs and edge cases | Input | Result | | --- | --- | | A union holding `NULL` (`union_value(k := NULL)`) | `NULL` | | `tag` is `NULL` | `NULL` | | A tag that isn't a member (`'m'`) | Error: `Could not find key "m" in union` | | A first argument that isn't a union | Error: `No function matches the given name and argument types 'union_extract(INTEGER_LITERAL, STRING_LITERAL)'` | The same cases, as the cluster returns them. They use the table and helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-utility-union-extract#set-up-the-examples): ``` run("SELECT union_extract(union_value(k := NULL), 'k'), union_extract(union_value(k := 1), NULL)") run("SELECT union_extract(union_value(k := 1)::UNION(k INTEGER, s VARCHAR), 'k'), " "union_extract(union_value(k := 1)::UNION(k INTEGER, s VARCHAR), 's')") run("SELECT union_extract(union_value(k := 1), 'm')") run("SELECT union_extract(1, 'k')") ``` ``` [(None, None)] [(1, None)] InternalError: Binder Error: Could not find key "m" in union InternalError: Binder Error: No function matches the given name and argument types 'union_extract(INTEGER_LITERAL, STRING_LITERAL)'. You might need to add explicit type casts. ``` ## Examples Wrap each numeric reading in a union and read it back. The result has the column's type, `BIGINT`: ``` run(f"SELECT id, union_extract(union_value(num := num), 'num'), " f"typeof(union_extract(union_value(num := num), 'num')) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 21, 'BIGINT'), (2, None, 'BIGINT'), (3, -4, 'BIGINT'), (4, None, 'BIGINT'), (5, None, 'BIGINT')] ``` One value per reading, a number or a status, of one `UNION(num BIGINT, status VARCHAR)` type; then the tag and both members of each. The member a row doesn't hold is `NULL`: ``` run(f"SELECT id, union_tag(r)::VARCHAR, union_extract(r, 'num'), union_extract(r, 'status') FROM (" "SELECT id, CASE WHEN num IS NOT NULL " "THEN union_value(num := num)::UNION(num BIGINT, status VARCHAR) " "ELSE union_value(status := status)::UNION(num BIGINT, status VARCHAR) END AS r " f"FROM {TABLE}) ORDER BY id LIMIT 10") ``` ``` [(1, 'num', 21, None), (2, 'status', None, 'offline'), (3, 'num', -4, None), (4, 'status', None, 'calibrating'), (5, 'status', None, None)] ``` Keep only the rows whose union holds a number: ``` run(f"SELECT id FROM {TABLE} WHERE union_extract(union_value(num := num), 'num') IS NOT NULL ORDER BY id LIMIT 10") ``` ``` [(1,), (3,)] ``` ## 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`). VAST DataBase stores no `UNION` columns and can't evaluate a function of a column, so `union_extract()` is always computed by the Query Engine: - **In the `SELECT` list,** the scan reads the columns and the Query Engine builds and reads the union for each row it returns. - **In `WHERE`,** the scan sends the columns for every row, and a `FILTER` step in the Query Engine computes `union_extract()` and keeps the rows that match. No rows are skipped in storage. The plan of the last example shows the `FILTER` step above the scan: ``` plan(f"SELECT id FROM {TABLE} WHERE union_extract(union_value(num := num), 'num') IS NOT NULL") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (union_extract(union_value│ │ (num), 'num') IS NOT NULL)│ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ num │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Filter on the raw columns.** `num IS NOT NULL` keeps the same rows as the last example without building a union. A condition on a raw column is pushed down to VAST DataBase, so rows that don't match never leave storage. Compare its plan with the one above: ``` plan(f"SELECT id FROM {TABLE} WHERE num IS NOT NULL") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ``` - **Build and read unions last,** in the `SELECT` list or a subquery, from rows already filtered on their columns. ## Limitations and errors - **The tag must be a member of the union's type.** Otherwise the query fails with `Could not find key "m" in union`. Cast the union to a type that lists the member, as in the examples, to read a member it may not hold. - **Unions only.** Any other first argument fails with `No function matches the given name and argument types`. For a struct, use [struct_extract](/develop/docs/dev-database-reference-sql-functions-structs-struct-extract). - **No `UNION` columns.** Union values exist only inside a query; see [union_value](/develop/docs/dev-database-reference-sql-functions-utility-union-value). ``` run("SELECT union_extract(struct_pack(k := 1), 'k')") run("SELECT struct_extract(struct_pack(k := 1), 'k')") ``` ``` InternalError: Binder Error: No function matches the given name and argument types 'union_extract(STRUCT(k INTEGER), STRING_LITERAL)'. You might need to add explicit type casts. [(1,)] ``` ## Similar functions | Function | Does | Example | Result | | --- | --- | --- | --- | | `union_extract` | Reads one member of a union | `union_extract(union_value(k := 1), 'k')` | `1` | | [union_value](/develop/docs/dev-database-reference-sql-functions-utility-union-value) | Builds a union with one member | `union_value(k := 1)` | `1`, a `UNION(k INTEGER)` | | `union_tag` | Returns the tag a union holds | `union_tag(union_value(k := 1))::VARCHAR` | `'k'` | | [struct_extract](/develop/docs/dev-database-reference-sql-functions-structs-struct-extract) | Reads one field of a struct | `struct_extract(struct_pack(k := 1), 'k')` | `1` | ``` run("SELECT union_extract(union_value(k := 1), 'k'), union_value(k := 1), union_tag(union_value(k := 1))::VARCHAR") run("SELECT struct_extract(struct_pack(k := 1), 'k')") ``` ``` [(1, 1, 'k')] [(1,)] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] ``` ## See also - [union_value](/develop/docs/dev-database-reference-sql-functions-utility-union-value). - [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs). - [Data types](/develop/docs/dev-database-reference-sql-data-types). - [Where filters run](/develop/docs/dev-database-reference-sql-select#where-filters-run) and [pushdowns](/develop/docs/dev-database-reference-pushdowns).