--- title: "struct_concat" slug: "dev-database-reference-sql-functions-structs-struct-concat" updated: 2026-10-30T18:19:01Z published: 2026-10-30T18:19:01Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-structs-struct-concat" --- > ## 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. # struct_concat *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) › Add fields.* Returns one struct with the fields of every struct you give, in order. ## Syntax ``` struct_concat(s1, s2, ...) ``` | Argument | Type | Description | | --- | --- | --- | | `s1`, `s2`, ... | `STRUCT` | The structs to join. Give at least one; their field names must all be different. | **Returns:** a `STRUCT` with the fields of `s1`, then those of `s2`, and so on: `typeof(struct_concat({'a': 1}, {'b': 2}))` is `STRUCT(a INTEGER, b INTEGER)`. ## 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_structs_struct_concat`. 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 books, each with a struct of details (`meta`), a shelf and a number of copies. Book 3 has an empty shelf and a struct of NULL fields, and book 4 has no values. `run()` also prints the error the Python client raises when it can't convert a result: ``` 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_structs_struct_concat"' 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.sub(r"^Parse 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, ValueError) 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}.books" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, meta STRUCT(lang VARCHAR, pages INTEGER), shelf VARCHAR, copies BIGINT)") run(f"INSERT INTO {TABLE} VALUES " "(1, {'lang': 'en', 'pages': 12}, 'A1', 3), " "(2, {'lang': 'de', 'pages': NULL}, 'B2', 1), " "(3, {'lang': NULL, 'pages': NULL}, '', 0), " "(4, NULL, NULL, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `struct_concat({'a': 1}, {'b': 2}, {'c': 3})` is `{'a': 1, 'b': 2, 'c': 3}`. The fields keep their types, so a field can be a list or another struct: `struct_concat({'a': {'x': 1}}, {'b': [1]})` is `{'a': {'x': 1}, 'b': [1]}`. With one struct, it returns that struct. It's the way to add the fields of a struct you already have, such as a struct column or a [struct_pack](/develop/docs/dev-database-reference-sql-functions-structs-struct-pack) result. To add fields one by one by name, [struct_insert](/develop/docs/dev-database-reference-sql-functions-structs-struct-insert) does the same. It doesn't change the table: it returns a new struct for each row. ## NULLs and edge cases | Input | Result | | --- | --- | | `struct_concat(NULL::STRUCT(a BIGINT), {'b': 1})` (a NULL struct) | `{'a': None, 'b': 1}`; the result isn't NULL | | `struct_concat({'a': 1}, NULL::STRUCT(b BIGINT))` | `{'a': 1, 'b': None}` | | `struct_concat({'a': 1})` | `{'a': 1}` | | `struct_concat({'a': 1}, NULL)` (an untyped NULL), or a string, number, list, map or date | Error: `Invalid Input Error: struct_concat: Argument at position "2" is not a STRUCT` (the position of the argument) | | `struct_concat({'a': 1}, {'a': 2})` | Error: `Invalid Input Error: struct_concat: Arguments contain duplicate STRUCT entry "a"` | | `struct_concat({'a': 1}, {'A': 2})` | Error: `Invalid Input Error: struct_concat: Arguments contain case-insensitive duplicate STRUCT entry "A" and "a"` | | `struct_concat({'a': 1}, row(2))` | Error: `Invalid Input Error: struct_concat: Cannot mix named and unnamed STRUCTs` | | `struct_concat()` | Error: `Invalid Input Error: struct_concat: At least one argument is required` | | `struct_concat({'a': 1}, {})` | Error: `Parser Error: syntax error at or near "}"`: there's no empty struct literal | The same cases, as the cluster returns them. They use the helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-structs-struct-concat#set-up-the-examples): ``` run("SELECT struct_concat(NULL::STRUCT(a BIGINT), {'b': 1}), struct_concat(NULL::STRUCT(a BIGINT), {'b': 1}) IS NULL, " "struct_concat({'a': 1}, NULL::STRUCT(b BIGINT)), struct_concat({'a': 1})") run("SELECT struct_concat({'a': 1}, NULL)") run("SELECT struct_concat('abc', {'b': 1})") run("SELECT struct_concat({'a': 1}, {'a': 2})") run("SELECT struct_concat({'a': 1}, {'A': 2})") run("SELECT struct_concat({'a': 1}, row(2))") run("SELECT struct_concat()") run("SELECT struct_concat({'a': 1}, {})") ``` ``` [({'a': None, 'b': 1}, False, {'a': 1, 'b': None}, {'a': 1})] InternalError: Invalid Input Error: struct_concat: Argument at position "2" is not a STRUCT InternalError: Invalid Input Error: struct_concat: Argument at position "1" is not a STRUCT InternalError: Invalid Input Error: struct_concat: Arguments contain duplicate STRUCT entry "a" InternalError: Invalid Input Error: struct_concat: Arguments contain case-insensitive duplicate STRUCT entry "A" and "a" InternalError: Invalid Input Error: struct_concat: Cannot mix named and unnamed STRUCTs InternalError: Invalid Input Error: struct_concat: At least one argument is required InternalError: Parser Error: syntax error at or near "}" ``` ## Examples Join each book's details with a struct of its shelf and copies. Book 4's `meta`, shelf and copies are NULL, so the result is a struct whose fields are all NULL, not a NULL struct: ``` run(f"SELECT id, struct_concat(meta, {{'shelf': shelf, 'copies': copies}}) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, {'lang': 'en', 'pages': 12, 'shelf': 'A1', 'copies': 3}), (2, {'lang': 'de', 'pages': None, 'shelf': 'B2', 'copies': 1}), (3, {'lang': None, 'pages': None, 'shelf': '', 'copies': 0}), (4, {'lang': None, 'pages': None, 'shelf': None, 'copies': None})] ``` Join three structs, one of them from [struct_pack](/develop/docs/dev-database-reference-sql-functions-structs-struct-pack): ``` run(f"SELECT id, struct_concat({{'id': id}}, meta, struct_pack(shelf := shelf)) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, {'id': 1, 'lang': 'en', 'pages': 12, 'shelf': 'A1'}), (2, {'id': 2, 'lang': 'de', 'pages': None, 'shelf': 'B2'}), (3, {'id': 3, 'lang': None, 'pages': None, 'shelf': ''}), (4, {'id': 4, 'lang': None, 'pages': None, 'shelf': None})] ``` Filter on a field of the result: ``` run(f"SELECT id FROM {TABLE} WHERE struct_concat(meta, {{'copies': copies}}).copies > 0 ORDER BY id LIMIT 10") ``` ``` [(1,), (2,)] ``` ## 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 can evaluate a comparison of a column, or of a struct field, with a constant, but not a function of a column, so `struct_concat()` is always computed by the Query Engine: - **In the `SELECT` list,** the scan reads the columns and the Query Engine builds the new struct for each row it returns. - **In `WHERE`,** the scan sends the columns for every row, and a `FILTER` step in the Query Engine computes the condition and keeps the rows that match. No rows are skipped in storage. The plan of the `WHERE` example shows the `FILTER` step above the scan: ``` plan(f"SELECT id FROM {TABLE} WHERE struct_concat(meta, {{'copies': copies}}).copies > 0") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #2 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (struct_extract │ │ (struct_concat(meta, │ │ struct_pack(copies)), │ │ 'copies') > 0) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ meta │ │ copies │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Filter on the stored column or field, not on the joined struct.** `copies > 0` and `meta.pages > 5` compare a column or a struct field with a constant, which VAST DataBase evaluates, so rows that don't match never leave storage. The plan has no `FILTER` step: ``` plan(f"SELECT id FROM {TABLE} WHERE copies > 0") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ``` - **Build the struct in the `SELECT` list.** Filter with the plain columns in `WHERE`, and join the structs only for the rows you return. ## Limitations and errors - **Field names must be different,** without regard to case. A struct joined with itself fails with `struct_concat: Arguments contain duplicate STRUCT entry "..."`. Rename the fields first, for example with a literal: `struct_concat(meta, {'lang2': meta.lang})`. - **Every argument must be a typed struct.** Anything else, including an untyped `NULL`, fails with `struct_concat: Argument at position "n" is not a STRUCT`. Cast a NULL: `NULL::STRUCT(b BIGINT)`. - **Named or unnamed, not both.** A struct from [row](/develop/docs/dev-database-reference-sql-functions-structs-row) can't be joined with a named struct: `Cannot mix named and unnamed STRUCTs`. Two unnamed structs can be joined, but the Python client can't read the result: `ValueError: Converting to Python dictionary is not supported when duplicate field names are present`. Check its type with `typeof()`, or read its fields by position. ``` run(f"SELECT id, struct_concat(meta, meta) FROM {TABLE} ORDER BY id LIMIT 10") run(f"SELECT id, struct_concat(meta, {{'lang2': meta.lang}}) FROM {TABLE} ORDER BY id LIMIT 10") run("SELECT struct_concat(row(1), row(2))") run("SELECT typeof(struct_concat(row(1), row(2))), struct_concat(row(1), row(2))[2]") ``` ``` InternalError: Invalid Input Error: struct_concat: Arguments contain duplicate STRUCT entry "lang" [(1, {'lang': 'en', 'pages': 12, 'lang2': 'en'}), (2, {'lang': 'de', 'pages': None, 'lang2': 'de'}), (3, {'lang': None, 'pages': None, 'lang2': None}), (4, {'lang': None, 'pages': None, 'lang2': None})] ValueError: Converting to Python dictionary is not supported when duplicate field names are present [('STRUCT(INTEGER, INTEGER)', 2)] ``` ## Similar functions | Function | Result for `{'a': 1}` and a field `b` of `'x'` | With a NULL struct `NULL::STRUCT(a BIGINT)` | | --- | --- | --- | | `struct_concat({'a': 1}, {'b': 'x'})` | `{'a': 1, 'b': 'x'}` | `{'a': None, 'b': 1}` for `{'b': 1}` | | `struct_concat({'a': 1}, struct_pack(b := 'x'))` | `{'a': 1, 'b': 'x'}` | | | [struct_insert](/develop/docs/dev-database-reference-sql-functions-structs-struct-insert): `struct_insert({'a': 1}, b := 'x')` | `{'a': 1, 'b': 'x'}` | `{'a': None, 'b': 1}` for `b := 1` | ``` run("SELECT struct_concat({'a': 1}, {'b': 'x'}), struct_concat({'a': 1}, struct_pack(b := 'x')), " "struct_insert({'a': 1}, b := 'x')") run("SELECT struct_concat(NULL::STRUCT(a BIGINT), {'b': 1}), struct_insert(NULL::STRUCT(a BIGINT), b := 1)") ``` ``` [({'a': 1, 'b': 'x'}, {'a': 1, 'b': 'x'}, {'a': 1, 'b': 'x'})] [({'a': None, 'b': 1}, {'a': None, 'b': 1})] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] ``` ## See also - [struct_insert](/develop/docs/dev-database-reference-sql-functions-structs-struct-insert): add named fields one by one. - [struct_pack](/develop/docs/dev-database-reference-sql-functions-structs-struct-pack) and [struct_extract](/develop/docs/dev-database-reference-sql-functions-structs-struct-extract): build a struct and read its fields. - [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs): all the struct functions. - [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.