--- title: "struct_pack" slug: "dev-database-reference-sql-functions-structs-struct-pack" updated: 2026-10-30T18:19:01Z published: 2026-10-30T18:19:01Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-structs-struct-pack" --- > ## 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_pack *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) › Build a struct.* Returns a struct with the named fields you give it, each with the type of its value. The literal `{'name': value, ...}` builds the same struct. ## Syntax ``` struct_pack(name := value, ...) struct_pack(name => value, ...) {'name': value, ...} ``` | Argument | Type | Description | | --- | --- | --- | | `name := value` | a name and a value of any type | One field: its name and its value. Names are unique, ignoring case. | **Returns:** `STRUCT(name type, ...)`, with the fields in the order you give them. **Aliases:** the `{'name': value, ...}` literal. ## 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_pack`. 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 rows with a struct column `st` (fields `a INTEGER` and `b VARCHAR`), a string `k` and a list `ks`. Row 4 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_structs_struct_pack"' 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}.items" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, st STRUCT(a INTEGER, b VARCHAR), k VARCHAR, ks VARCHAR[])") run(f"INSERT INTO {TABLE} VALUES " "(1, {'a': 1, 'b': 'x'}, 'de', ['en', 'de']), " "(2, {'a': -7, 'b': NULL}, 'fr', ['\u00e9', '\u00fc']), " "(3, {'a': NULL, 'b': NULL}, '', []), " "(4, NULL, NULL, NULL), " "(5, {'a': 2147483647, 'b': ''}, 'k3', ['k1', 'k2', 'k3'])") ``` ``` [] [] [(5,)] ``` ## What it does `struct_pack` makes one struct per row from the values you name: - **Each argument is a field.** `struct_pack(a := 1, b := 'x')` is `{'a': 1, 'b': 'x'}`, of type `STRUCT(a INTEGER, b VARCHAR)`. `=>` works in place of `:=`. - **Any value type,** including lists, maps, dates and other structs: `struct_pack(a := struct_pack(b := 1))` is `{'a': {'b': 1}}`. Python receives a struct as a `dict`. - **The literal is the same struct.** `struct_pack(a := 1) = {'a': 1}` is true. In the literal, a name can be quoted or not: `{a: 1}` is `{'a': 1}`. - **Names that aren't plain words** are quoted: `struct_pack("a b" := 1)` has a field `a b`. - **A cast reshapes a struct by field name.** `{'a': 1, 'b': 2}::STRUCT(b BIGINT, a BIGINT)` reorders the fields, a field the target type adds is NULL, and a field it leaves out is dropped. At least one name must match. ## NULLs and edge cases | Input | Result | | --- | --- | | `struct_pack(a := NULL)` | `{'a': None}`: a struct with a NULL field, not a NULL struct | | `struct_pack(a := NULL) IS NULL` | `false` | | `struct_pack(a := NULL, b := NULL)` | `{'a': None, 'b': None}` | | `{'a': 1}::STRUCT(a BIGINT, b BIGINT)` (a field added) | `{'a': 1, 'b': None}` | | `{'a': 1, 'b': 2}::STRUCT(a BIGINT)` (a field dropped) | `{'a': 1}` | | `{'a': 1, 'b': 2}::STRUCT(b BIGINT, a BIGINT)` | `{'b': 2, 'a': 1}` | | `struct_pack(a := 1, A := 2)` (the same name, ignoring case) | Error: `Duplicate struct entry name "A"` | | `struct_pack()` | Error: `Can't pack nothing into a struct` | | `{'a': 1}::STRUCT(z BIGINT)` (no name matches) | Error: `STRUCT to STRUCT cast must have at least one matching member` | 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-structs-struct-pack#set-up-the-examples): ``` run("SELECT struct_pack(a := NULL), struct_pack(a := NULL) IS NULL, struct_pack(a := NULL, b := NULL)") run("SELECT {'a': 1}::STRUCT(a BIGINT, b BIGINT), {'a': 1, 'b': 2}::STRUCT(a BIGINT), " "{'a': 1, 'b': 2}::STRUCT(b BIGINT, a BIGINT)") run("SELECT struct_pack(a := 1, A := 2)") run("SELECT struct_pack()") run("SELECT {'a': 1}::STRUCT(z BIGINT)") ``` ``` [({'a': None}, False, {'a': None, 'b': None})] [({'a': 1, 'b': None}, {'a': 1}, {'b': 2, 'a': 1})] InternalError: Binder Error: Duplicate struct entry name "A" InternalError: Invalid Input Error: Can't pack nothing into a struct InternalError: Binder Error: STRUCT to STRUCT cast must have at least one matching member ``` ## Examples A struct per row from other columns, with `struct_pack` and with the literal. A NULL value gives a NULL field, so row 4 still gets a struct: ``` run(f"SELECT id, struct_pack(id := id, label := st.b), {{'id': id, 'n': len(ks)}} FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, {'id': 1, 'label': 'x'}, {'id': 1, 'n': 2}), (2, {'id': 2, 'label': None}, {'id': 2, 'n': 2}), (3, {'id': 3, 'label': None}, {'id': 3, 'n': 0}), (4, {'id': 4, 'label': None}, {'id': 4, 'n': None}), (5, {'id': 5, 'label': ''}, {'id': 5, 'n': 3})] ``` Write structs to a struct column. This block creates a second table, `copies`, and inserts a struct built by `struct_pack` from the first table, a literal with a missing field, and a literal with an extra field. The read-back shows what each row stores: ``` COPIES = f"{SCHEMA}.copies" run(f"CREATE TABLE IF NOT EXISTS {COPIES} (id BIGINT, st STRUCT(a INTEGER, b VARCHAR))") run(f"INSERT INTO {COPIES} SELECT id, struct_pack(a := st.a, b := k) FROM {TABLE} WHERE id = 1") run(f"INSERT INTO {COPIES} VALUES (2, {{'a': 1}}), (3, {{'a': 1, 'b': 'x', 'c': 2}})") run(f"SELECT id, st FROM {COPIES} ORDER BY id LIMIT 10") ``` ``` [] [(1,)] [(2,)] [(1, {'a': 1, 'b': 'de'}), (2, {'a': 1, 'b': None}), (3, {'a': 1, 'b': '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`). `struct_pack` builds its struct in the Query Engine: - **In the `SELECT` list,** the scan reads the columns the fields use, and the Query Engine builds the struct for each row it returns. - **In `WHERE`,** a condition on a struct you build, such as `struct_pack(a := st.a) = {'a': 1}`, runs in a `FILTER` step in the Query Engine: the scan sends the columns for every row. No rows are skipped in storage. The plan of that condition shows the `FILTER` step above the scan: ``` run(f"SELECT id FROM {TABLE} WHERE struct_pack(a := st.a) = {{'a': 1}} ORDER BY id LIMIT 10") plan(f"SELECT id FROM {TABLE} WHERE struct_pack(a := st.a) = {{'a': 1}}") ``` ``` [(1,)] ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (struct_pack │ │ (struct_extract(st, 'a')) │ │ = {'a': 1}) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ st.a │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Compare fields, not structs you build.** `st.a = 1` returns the same rows as `struct_pack(a := st.a) = {'a': 1}`, and it's evaluated in VAST DataBase: its plan has no `FILTER` step: ``` run(f"SELECT id FROM {TABLE} WHERE st.a = 1 ORDER BY id LIMIT 10") plan(f"SELECT id FROM {TABLE} WHERE st.a = 1") ``` ``` [(1,)] ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ``` ## Limitations and errors - **Every argument needs a name.** `struct_pack(42)` fails with `Need named argument for struct pack`. An empty struct can't be made: `struct_pack()` fails with `Can't pack nothing into a struct`, and `{}` with a syntax error. - **Names are unique, ignoring case.** `struct_pack(a := 1, A := 2)` and `{'a': 1, 'a': 2}` fail with `Duplicate struct entry name`. - **Writing to a struct column matches fields by name.** A struct whose names don't match any field of the column fails with `STRUCT to STRUCT cast must have at least one matching member`; a value that doesn't convert to the field's type fails with a conversion error, such as `Could not convert string 'notint' to INT32`. - **Other SQL dialects' forms aren't available.** `named_struct('a', 1)` fails with `Scalar Function with name named_struct does not exist!`, and `struct(1 AS a)` and `ROW(1 AS a)` with a syntax error. Use `struct_pack(a := 1)` or `{'a': 1}`. ``` run("SELECT struct_pack(42)") run("SELECT {'a': 1, 'a': 2}") run(f"INSERT INTO {COPIES} VALUES (4, {{'x': 1, 'y': 'x'}})") run(f"INSERT INTO {COPIES} VALUES (5, {{'a': 'notint', 'b': 'x'}})") run("SELECT named_struct('a', 1, 'b', 'x')") ``` ``` InternalError: Binder Error: Need named argument for struct pack, e.g. STRUCT_PACK(a := b) InternalError: Binder Error: Duplicate struct entry name "a" InternalError: Binder Error: STRUCT to STRUCT cast must have at least one matching member InternalError: Conversion Error: Could not convert string 'notint' to INT32 InternalError: Catalog Error: Scalar Function with name named_struct does not exist! ``` ## Similar functions | Function | Builds | Result | | --- | --- | --- | | `struct_pack(a := 1, b := 'x')`, `{'a': 1, 'b': 'x'}` | a struct with named fields | `{'a': 1, 'b': 'x'}` | | [row](/develop/docs/dev-database-reference-sql-functions-structs-row)`(1, 'x')` | a struct with unnamed fields, type `STRUCT(INTEGER, VARCHAR)` | read it with `[i]` or `to_json`: `'{"":1,"":"x"}'` | | [struct_insert](/develop/docs/dev-database-reference-sql-functions-structs-struct-insert)`({'a': 1}, b := 'x')` | a struct with fields added | `{'a': 1, 'b': 'x'}` | | [struct_concat](/develop/docs/dev-database-reference-sql-functions-structs-struct-concat)`({'a': 1}, {'b': 'x'})` | one struct from the fields of several | `{'a': 1, 'b': 'x'}` | ``` run("SELECT struct_pack(a := 1, b := 'x'), {'a': 1, 'b': 'x'}, typeof(row(1, 'x')), to_json(row(1, 'x'))") run("SELECT struct_insert({'a': 1}, b := 'x'), struct_concat({'a': 1}, {'b': 'x'})") ``` ``` [({'a': 1, 'b': 'x'}, {'a': 1, 'b': 'x'}, 'STRUCT(INTEGER, VARCHAR)', '{"":1,"":"x"}')] [({'a': 1, 'b': 'x'}, {'a': 1, 'b': 'x'})] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {COPIES}") run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] [] ``` ## See also - [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs): all the struct functions. - [struct_extract](/develop/docs/dev-database-reference-sql-functions-structs-struct-extract): read a field. - [Data types in SQL](/develop/docs/dev-database-reference-sql-data-types): struct columns. - [Where filters run](/develop/docs/dev-database-reference-sql-select#where-filters-run) and [pushdowns](/develop/docs/dev-database-reference-pushdowns).