--- title: "array_to_json" slug: "dev-database-reference-sql-functions-json-array-to-json" updated: 2026-10-30T18:19:06Z published: 2026-10-30T18:19:06Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-json-array-to-json" --- > ## 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. # array_to_json *Applies to VAST Cluster 5.5.* *[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [Parsing and JSON functions](/develop/docs/dev-database-reference-sql-functions-json) › Build JSON.* Converts a list to a JSON array. It takes only a list; for other values, use [to_json](/develop/docs/dev-database-reference-sql-functions-json-to-json). ## Syntax ``` array_to_json(list) ``` | Argument | Type | Description | | --- | --- | --- | | `list` | a list of any element type | The list to convert. | **Returns:** `JSON` (text; the driver returns it as a string). ## 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_json_array_to_json`. 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 rows with two list columns. Row 2 has empty lists, row 3 a `NULL` element, and row 4 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_json_array_to_json"' 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, scores BIGINT[], tags VARCHAR[])") run(f"INSERT INTO {TABLE} VALUES " "(1, [1, 2, 3], ['math', 'computing']), " "(2, [], []), " "(3, [NULL, 4], ['say \"hi\"', NULL]), " "(4, NULL, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `array_to_json` writes the list as a JSON array, converting each element as [to_json](/develop/docs/dev-database-reference-sql-functions-json-to-json) does: | Elements | Example | Result | | --- | --- | --- | | Numbers | `array_to_json([1, 2, 3])` | `[1,2,3]` | | Strings, quoted and escaped | `array_to_json(['a', 'é'])` | `["a","é"]` | | `NULL` | `array_to_json([NULL])` | `[null]` | | Lists | `array_to_json([[1, 2], [3]])` | `[[1,2],[3]]` | | Structs | `array_to_json([{'a': 1}])` | `[{"a":1}]` | | Dates | `array_to_json([DATE '2026-10-16'])` | `["2026-10-16"]` | | `JSON` values, embedded as documents | `array_to_json(['{"a": 1}'::JSON])` | `[{"a":1}]` | | `VARCHAR` values holding JSON, quoted as strings | `array_to_json(['{"a": 1}'])` | `["{\"a\": 1}"]` | The result is compact, with no spaces. ## NULLs and edge cases | Input | Result | | --- | --- | | `NULL` | `NULL` | | `[]` (empty list) | `[]` | | `[1.5, 'NaN'::DOUBLE]` | `[1.5,NaN]`: `NaN` isn't part of the JSON standard | | A number, string, struct, date, or JSON text such as `'[1, 2]'` | Error: `Invalid Input Error: array_to_json() argument type must be LIST` | | A fixed-size array, `[1, 2]::INTEGER[2]` | The same error | The cases of both tables, as the cluster returns them. Each line shows the argument and the result. They use the helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-json-array-to-json#set-up-the-examples): ``` for expr in ["[1, 2, 3]", "['a', '\u00e9']", "[NULL]", "[[1, 2], [3]]", "[{'a': 1}]", "[DATE '2026-10-16']", "['{\"a\": 1}'::JSON]", "['{\"a\": 1}']", "NULL", "[]", "[1.5, 'NaN'::DOUBLE]", "123", "'abc'", "{'a': 1}", "DATE '2026-10-16'", "'[1, 2]'", "[1, 2]::INTEGER[2]"]: print(expr, end=" ") run(f"SELECT array_to_json({expr})") ``` ``` [1, 2, 3] [('[1,2,3]',)] ['a', 'é'] [('["a","é"]',)] [NULL] [('[null]',)] [[1, 2], [3]] [('[[1,2],[3]]',)] [{'a': 1}] [('[{"a":1}]',)] [DATE '2026-10-16'] [('["2026-10-16"]',)] ['{"a": 1}'::JSON] [('[{"a":1}]',)] ['{"a": 1}'] [('["{\\"a\\": 1}"]',)] NULL [(None,)] [] [('[]',)] [1.5, 'NaN'::DOUBLE] [('[1.5,NaN]',)] 123 InternalError: Invalid Input Error: array_to_json() argument type must be LIST 'abc' InternalError: Invalid Input Error: array_to_json() argument type must be LIST {'a': 1} InternalError: Invalid Input Error: array_to_json() argument type must be LIST DATE '2026-10-16' InternalError: Invalid Input Error: array_to_json() argument type must be LIST '[1, 2]' InternalError: Invalid Input Error: array_to_json() argument type must be LIST [1, 2]::INTEGER[2] InternalError: Invalid Input Error: array_to_json() argument type must be LIST ``` ## Examples Convert both list columns. Row 3's `NULL` elements become `null`, and the double quotes are escaped: ``` run(f"SELECT id, array_to_json(scores), array_to_json(tags) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, '[1,2,3]', '["math","computing"]'), (2, '[]', '[]'), (3, '[null,4]', '["say \\"hi\\"",null]'), (4, None, None)] ``` ## 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`). `array_to_json` runs in the Query Engine: the scan reads the list column, and a projection step above it builds the JSON for each row it returns. In `WHERE`, a condition on `array_to_json()` runs in a `FILTER` step in the Query Engine. ``` plan(f"SELECT id, array_to_json(scores) FROM {TABLE}") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ id │ │ array_to_json(scores) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ id │ │ scores │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Filter before you build.** A comparison of a plain column with a constant, such as `id = 1`, is pushed down to VAST DataBase, so only the rows you return reach `array_to_json`. - **Filter on the list, not on its JSON.** [List functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs) work on the list column directly, with no conversion to text. ## Limitations and errors - **Lists only.** Anything else, including JSON text and fixed-size arrays, fails with `array_to_json() argument type must be LIST`. Use [to_json](/develop/docs/dev-database-reference-sql-functions-json-to-json) for other values. - **No aggregation into an array.** `array_to_json(list(id))`, which would build one array from many rows, fails with `The operator 'HASH_GROUP_BY' type=9 is currently not supported.` Read the rows with your client and build the array there. ``` run(f"SELECT array_to_json(list(id)) FROM {TABLE}") ``` ``` InternalError: The operator 'HASH_GROUP_BY' type=9 is currently not supported. ``` ## Similar functions | Function | Takes | `[1, 2]` | `{'a': 1}` | | --- | --- | --- | --- | | `array_to_json` | a list | `[1,2]` | (error) | | [to_json](/develop/docs/dev-database-reference-sql-functions-json-to-json) | any value | `[1,2]` | `{"a":1}` | | [json_quote](/develop/docs/dev-database-reference-sql-functions-json-json-quote) | any value | `[1,2]` | `{"a":1}` | | [json_array](/develop/docs/dev-database-reference-sql-functions-json-json-array) | the elements as arguments | `json_array(1, 2)` gives `[1,2]` | | | [row_to_json](/develop/docs/dev-database-reference-sql-functions-json-row-to-json) | a struct | (error) | `{"a":1}` | ``` run("SELECT array_to_json([1, 2]), to_json([1, 2]), json_quote([1, 2]), json_array(1, 2), " "to_json({'a': 1}), json_quote({'a': 1}), row_to_json({'a': 1})") run("SELECT array_to_json({'a': 1})") run("SELECT row_to_json([1, 2])") ``` ``` [('[1,2]', '[1,2]', '[1,2]', '[1,2]', '{"a":1}', '{"a":1}', '{"a":1}')] InternalError: Invalid Input Error: array_to_json() argument type must be LIST InternalError: Invalid Input Error: row_to_json() argument type must be STRUCT ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] ``` ## See also - [to_json](/develop/docs/dev-database-reference-sql-functions-json-to-json): the conversion rules for each element type. - [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs). - [Parsing and JSON functions](/develop/docs/dev-database-reference-sql-functions-json): all the JSON functions.