--- title: "json_deserialize_sql" slug: "dev-database-reference-sql-functions-json-json-deserialize-sql" updated: 2026-10-30T18:19:06Z published: 2026-10-30T18:19:06Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-json-json-deserialize-sql" --- > ## 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_deserialize_sql *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) › SQL as JSON.* Turns the JSON syntax tree of a `SELECT` statement, as [json_serialize_sql](/develop/docs/dev-database-reference-sql-functions-json-json-serialize-sql) returns it, back into SQL text. A round trip through both functions writes a query in a normalised form. ## Syntax ``` json_deserialize_sql(tree) ``` | Argument | Type | Description | | --- | --- | --- | | `tree` | `JSON` or `VARCHAR` | The result of `json_serialize_sql` for one `SELECT` statement, written without the `skip_null` and `skip_empty` options. | **Returns:** `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_json_json_deserialize_sql`. 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 saved queries: three `SELECT` statements written in different styles, 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_deserialize_sql"' 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}.saved_queries" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, q VARCHAR)") run(f"INSERT INTO {TABLE} VALUES " "(1, 'SELECT name FROM people WHERE id = 1'), " "(2, 'select a+1 from t where b = ''x'' order by 1 limit 2'), " "(3, 'WITH a AS (SELECT 1 AS x) SELECT x FROM a'), " "(4, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `json_deserialize_sql` writes the statement in the tree as SQL text. The text is rebuilt from the tree, not copied, so it's normalised: - **Keywords are in capitals:** `select` becomes `SELECT`. - **Expressions are in parentheses:** `a+1` becomes `(a + 1)`, and `b = 'x'` becomes `(b = 'x')`. - **A column name such as `name`** is written in double quotes: `SELECT "name" FROM people`. - **A `WITH` clause** is written without a space before the main `SELECT`: `WITH a AS (SELECT 1 AS x)SELECT x FROM a`. ``` run("SELECT json_deserialize_sql(json_serialize_sql('SELECT 1 AS x'))") run("SELECT json_deserialize_sql(json_serialize_sql('select a+1 from t where b = ''x'' order by 1 limit 2'))") ``` ``` [('SELECT 1 AS x',)] [("SELECT (a + 1) FROM t WHERE (b = 'x') ORDER BY 1 LIMIT 2",)] ``` ## NULLs and edge cases | Input | Result | | --- | --- | | `NULL`, or the tree of `NULL` | `NULL` | | The tree of two statements | Error: `Error parsing json: more than one statement` | | The tree of text that isn't SQL | Error: `Error parsing json: parser: syntax error at or near "not"` (for `'not sql'`) | | A tree written with `skip_null` and `skip_empty` | Error: `Expected but did not find property 'cte_map' in json object: ...` | | `'{}'` | Error: `Error parsing json: no statements array` | | `'{"statements": []}'` | Error: `Error parsing json: no statements` | | `'not json'` | Error: `Malformed JSON at byte 0 of input: invalid literal.` | ``` run("SELECT json_deserialize_sql(NULL), json_deserialize_sql(json_serialize_sql(NULL))") run("SELECT json_deserialize_sql(json_serialize_sql('SELECT 1; SELECT 2'))") run("SELECT json_deserialize_sql(json_serialize_sql('not sql'))") run("SELECT json_deserialize_sql(json_serialize_sql('SELECT 1', skip_null := true, skip_empty := true))") run("SELECT json_deserialize_sql('{}')") run("SELECT json_deserialize_sql('{\"statements\": []}')") run("SELECT json_deserialize_sql('not json')") ``` ``` [(None, None)] OSError: Error parsing json: more than one statement OSError: Error parsing json: parser: syntax error at or near "not" OSError: Expected but did not find property 'cte_map' in json object: '{"type":"SELECT_NODE","select_list":[{"class":"CONSTANT","type":"VALUE_CONSTANT","query_location":7,"value":{"type":{"id":"INTEGER"},"is_null":false,"value":1}}],"from_table":{"type":"EMPTY","query_location":18446744073709551615},"aggregate_handling":"STANDARD_HANDLING"}' OSError: Error parsing json: no statements array OSError: Error parsing json: no statements OSError: Malformed JSON at byte 0 of input: invalid literal. Input: not json ``` ## Examples Each saved query in normalised form. The NULL row stays NULL: ``` run(f"SELECT id, json_deserialize_sql(json_serialize_sql(q)) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 'SELECT "name" FROM people WHERE (id = 1)'), (2, "SELECT (a + 1) FROM t WHERE (b = 'x') ORDER BY 1 LIMIT 2"), (3, 'WITH a AS (SELECT 1 AS x)SELECT x FROM a'), (4, None)] ``` A row whose text isn't a single `SELECT` statement fails the whole query. Add one, then keep only the rows that `json_serialize_sql` parsed without an error: ``` run(f"INSERT INTO {TABLE} VALUES (5, 'not sql')") run(f"SELECT id, json_deserialize_sql(json_serialize_sql(q)) FROM {TABLE} ORDER BY id LIMIT 10") run(f"SELECT id, json_deserialize_sql(json_serialize_sql(q)) FROM {TABLE} " f"WHERE json_serialize_sql(q) ->> '$.error' = 'false' ORDER BY id LIMIT 10") ``` ``` [(1,)] OSError: Error parsing json: parser: syntax error at or near "not" [(1, 'SELECT "name" FROM people WHERE (id = 1)'), (2, "SELECT (a + 1) FROM t WHERE (b = 'x') ORDER BY 1 LIMIT 2"), (3, 'WITH a AS (SELECT 1 AS x)SELECT x FROM a')] ``` ## How it runs in VAST The Query Engine runs on the cluster's CNodes, and a query on a table starts with a scan in VAST DataBase (`VAST_SCAN`). `json_deserialize_sql` reads the tree and writes the text in the Query Engine; nothing is run. In the `SELECT` list, the scan reads the column and a projection step computes the text for each row it returns: ``` plan(f"SELECT json_deserialize_sql(json_serialize_sql(q)) FROM {TABLE}") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ json_deserialize_sql │ │ (json_serialize_sql(q)) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: q │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Normalise when you write.** Store the normalised text in its own column when you save a query. Then a comparison of that column with a constant is evaluated in VAST DataBase, without parsing any text. - **Select the rows with a condition on a column,** such as `id = 1`, which VAST DataBase evaluates, so only those rows are parsed. ## Limitations and errors - **One `SELECT` statement per tree.** A tree of several statements fails with `Error parsing json: more than one statement`; split the text into statements first. - **Full trees only.** A tree written with `skip_null` and `skip_empty` fails with `Expected but did not find property ...`. Serialize without those options. - **An error object fails the query.** If `json_serialize_sql` couldn't parse the text, `json_deserialize_sql` fails with `Error parsing json:` and the parser's message. Check `json_serialize_sql(q) ->> '$.error'` first, as in the second example. - **The original layout isn't kept,** because the text is rebuilt from the tree. ## Similar functions | Function | What it does | | --- | --- | | [json_serialize_sql](/develop/docs/dev-database-reference-sql-functions-json-json-serialize-sql) | SQL text to a JSON syntax tree | | `json_deserialize_sql` | a JSON syntax tree back to SQL text | ``` run("SELECT json_serialize_sql('SELECT 1 AS x') ->> '$.error', " "json_deserialize_sql(json_serialize_sql('SELECT 1 AS x'))") ``` ``` [('false', 'SELECT 1 AS x')] ``` 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_serialize_sql](/develop/docs/dev-database-reference-sql-functions-json-json-serialize-sql). - [EXPLAIN](/develop/docs/dev-database-reference-sql-explain): how to read a plan.