--- title: "json_merge_patch" slug: "dev-database-reference-sql-functions-json-json-merge-patch" updated: 2026-10-30T18:19:06Z published: 2026-10-30T18:19:06Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-json-json-merge-patch" --- > ## 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. # json_merge_patch *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.* Applies one or more merge patches to a JSON document: keys in a patch are added or replace the document's, nested objects are merged, and a key set to `null` is removed. Patches are applied from left to right. ## Syntax ``` json_merge_patch(document, patch, ...) ``` | Argument | Type | Description | | --- | --- | --- | | `document` | `VARCHAR` or `JSON` | The JSON document to change. | | `patch` | `VARCHAR` or `JSON` | One or more patches, applied in order. | **Returns:** `JSON`. The driver returns it as text (a Python `str`). ## 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_json_merge_patch`. 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 user profiles stored as JSON text: two objects, an array (not a profile), and a 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_json_json_merge_patch"' 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}.profiles" PATCH = '{"n": null, "geo": {"lat": null}, "seen": true}' run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, doc VARCHAR)") run(f"INSERT INTO {TABLE} VALUES " "(1, '{\"user\": \"ada\", \"n\": 3, \"tags\": [\"a\", \"b\"], \"geo\": {\"city\": \"Oslo\", \"lat\": 59.9}}'), " "(2, '{\"user\": \"alan\", \"n\": \"many\", \"tags\": []}'), " "(3, '[10, 20]'), " "(4, NULL)") ``` ``` [] [] [(4,)] ``` `PATCH` removes `n` and the nested `geo.lat`, and adds `seen`. ## What it does For each patch in turn: - **A key in the patch** is added to the document, or replaces the document's value for that key. - **A nested object in the patch** is merged into the document's object for that key, by the same rules. - **A key set to `null`** is removed from the document. - **An array in the patch** replaces the document's array; arrays aren't merged. - **A patch that isn't an object,** such as `[1]` or `null`, replaces the whole document. | Document | Patch | Result | | --- | --- | --- | | `{"a": 1}` | `{"b": 2}` | `{"a":1,"b":2}` | | `{"a": 1}` | `{"a": null}` | `{}` | | `{"a": {"b": 1, "c": 2}}` | `{"a": {"b": null}}` | `{"a":{"c":2}}` | | `{"a": [1, 2]}` | `{"a": [3]}` | `{"a":[3]}` | | `{"a": 1}` | `[1]` | `[1]` | | `{"a": 1}` | `{}` | `{"a":1}` | | `{"a": 1}` | `{"b": 2}`, then `{"a": 3}` | `{"b":2,"a":3}` | ``` run("SELECT json_merge_patch('{\"a\": 1}', '{\"b\": 2}'), json_merge_patch('{\"a\": 1}', '{\"a\": null}')") run("SELECT json_merge_patch('{\"a\": {\"b\": 1, \"c\": 2}}', '{\"a\": {\"b\": null}}'), " "json_merge_patch('{\"a\": [1, 2]}', '{\"a\": [3]}')") run("SELECT json_merge_patch('{\"a\": 1}', '[1]'), json_merge_patch('{\"a\": 1}', '{}'), " "json_merge_patch('{\"a\": 1}', '{\"b\": 2}', '{\"a\": 3}')") ``` ``` [('{"a":1,"b":2}', '{}')] [('{"a":{"c":2}}', '{"a":[3]}')] [('[1]', '{"a":1}', '{"b":2,"a":3}')] ``` ## NULLs and edge cases | Input | Result | | --- | --- | | A NULL patch | `NULL` | | A NULL document | the patch, as written | | A document that isn't an object (an array, a string, a number, `null`) | the patch, as written: its `null` members are kept | | `'{}'` with patch `{"a": {"b": null}}` | `{"a":{}}` | | Patch `'null'` | `null` | | One argument, or none | Error: `json_merge_patch requires at least two parameters` | ``` run("SELECT json_merge_patch('{\"a\": 1}', NULL), json_merge_patch(NULL, '{\"a\": 1, \"b\": null}')") run("SELECT json_merge_patch('[1]', '{\"a\": 1, \"b\": null}'), json_merge_patch('{}', '{\"a\": {\"b\": null}}'), " "json_merge_patch('{\"a\": 1}', 'null')") run("SELECT json_merge_patch('{\"a\": 1}')") ``` ``` [(None, '{"a":1,"b":null}')] [('{"a":1,"b":null}', '{"a":{}}', 'null')] InternalError: Invalid Input Error: json_merge_patch requires at least two parameters ``` ## Examples Apply `PATCH` to every profile. The objects in rows 1 and 2 lose `n` and gain `seen`. Row 1 also loses `geo.lat`, and row 2, which has no `geo`, gains an empty `geo` object. The array and the NULL row give the patch as written: ``` run(f"SELECT id, json_merge_patch(doc, '{PATCH}') FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, '{"user":"ada","tags":["a","b"],"geo":{"city":"Oslo"},"seen":true}'), (2, '{"user":"alan","tags":[],"geo":{},"seen":true}'), (3, '{"n":null,"geo":{"lat":null},"seen":true}'), (4, '{"n":null,"geo":{"lat":null},"seen":true}')] ``` Patch only the objects, and leave the other rows as they are: ``` run(f"SELECT id, CASE WHEN json_type(doc) = 'OBJECT' THEN json_merge_patch(doc, '{PATCH}') ELSE doc END " f"FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, '{"user":"ada","tags":["a","b"],"geo":{"city":"Oslo"},"seen":true}'), (2, '{"user":"alan","tags":[],"geo":{},"seen":true}'), (3, '[10, 20]'), (4, 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`). VAST DataBase stores JSON as text and doesn't parse it, so `json_merge_patch` is always computed by the Query Engine: - **In the `SELECT` list,** the scan reads the whole document and the Query Engine parses and patches it for each row it returns. - **In `WHERE`,** a `FILTER` step in the Query Engine patches the document of every row the scan sends. ``` plan(f"SELECT id FROM {TABLE} WHERE json_merge_patch(doc, '{PATCH}') IS NOT NULL") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (json_merge_patch(doc, '{ │ │ "n": null, "geo": {"lat": │ │ null}, "seen": true}') IS│ │ NOT NULL) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ doc │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Select the rows with a condition on a column.** A comparison such as `id = 1` is evaluated in VAST DataBase, so only those documents are parsed and patched. The plan has no `FILTER` step: ``` plan(f"SELECT json_merge_patch(doc, '{PATCH}') FROM {TABLE} WHERE id = 1") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ json_merge_patch(doc, '{"n│ │ ": null, "geo": {"lat": │ │ null}, "seen": true}') │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: doc │ │ │ └───────────────────────────┘ ``` ## Limitations and errors - **Text or `JSON` only.** A number, a date, a boolean, a list or a `BLOB` as the document, or a number as a patch, fails with `Invalid Input Error: Arguments to json_merge_patch must be of type VARCHAR or JSON`. Convert the value with [to_json](/develop/docs/dev-database-reference-sql-functions-json-to-json) first. - **At least two arguments.** `json_merge_patch(doc)` fails with `json_merge_patch requires at least two parameters`. - **Invalid JSON fails the query** with `Malformed JSON at byte N of input: ...`. Filter the rows with [json_valid](/develop/docs/dev-database-reference-sql-functions-json-json-valid) first. - **A document that isn't an object is replaced by the patch,** `null` members included. Check the document with [json_type](/develop/docs/dev-database-reference-sql-functions-json-json-type) if the column can hold arrays or scalars, as in the second example. - **Repeated keys in the document are kept,** each with its own value. ``` run("SELECT json_merge_patch(to_json([1, 2]), '{\"a\": 1}')") run("SELECT json_merge_patch([1, 2], '{\"a\": 1}')") run("SELECT json_merge_patch('not json', '{}')") ``` ``` [('{"a":1}',)] InternalError: Invalid Input Error: Arguments to json_merge_patch must be of type VARCHAR or JSON OSError: Malformed JSON at byte 0 of input: invalid literal. Input: not json ``` ## Similar functions | Function | What it does | Result | | --- | --- | --- | | `json_merge_patch('{"a": 1}', '{"b": 2}')` | merges documents | `{"a":1,"b":2}` | | [json_object](/develop/docs/dev-database-reference-sql-functions-json-json-object)`('a', 1, 'b', 2)` | builds an object from pairs | `{"a":1,"b":2}` | ``` run("SELECT json_merge_patch('{\"a\": 1}', '{\"b\": 2}'), json_object('a', 1, 'b', 2)") ``` ``` [('{"a":1,"b":2}', '{"a":1,"b":2}')] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] ``` ## See also - [Parsing and JSON functions](/develop/docs/dev-database-reference-sql-functions-json): all the JSON functions. - [json_object](/develop/docs/dev-database-reference-sql-functions-json-json-object) and [json_type](/develop/docs/dev-database-reference-sql-functions-json-json-type). - [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.