--- title: "url_decode" slug: "dev-database-reference-sql-functions-string-url-decode" updated: 2026-10-30T18:18:56Z published: 2026-10-30T18:18:56Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-string-url-decode" --- > ## 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. # url_decode *Applies to VAST Cluster 5.5.* *[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [Text basics](/develop/docs/dev-database-reference-sql-functions-string-text-basics) › Unicode and URLs.* Decodes a percent-encoded string: each `%` followed by two hexadecimal digits becomes the byte it stands for, and the bytes are read as UTF-8. It reverses [url_encode](/develop/docs/dev-database-reference-sql-functions-string-url-encode). It has no aliases. ## Syntax ``` url_decode(s) ``` | Argument | Type | Description | | --- | --- | --- | | `s` | `VARCHAR` | The percent-encoded string to decode. | **Returns:** `VARCHAR`. **Aliases:** none. ## 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_url_decode`. 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 query-string values from a web server's access log, still percent-encoded. Row 3 came from a form, so its space is a `+`. The last row has no value: ``` 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_url_decode"' 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}.requests" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, param VARCHAR, value VARCHAR)") run(f"INSERT INTO {TABLE} VALUES " "(1, 'q', 'caf%C3%A9%20menu'), " "(2, 'path', '%2Fdata%2Flogs%2Fapp.log'), " "(3, 'name', 'Ada+Lovelace'), " "(4, 'q', 'R%26D'), " "(5, 'q', NULL)") ``` ``` [] [] [(5,)] ``` ## What it does `url_decode` reads the string from left to right. Each `%` followed by two hexadecimal digits is replaced by the byte with that value; everything else is copied as it is. The resulting bytes must be valid UTF-8: | Call | Result | | --- | --- | | `url_decode('a%20b%26c')` | `'a b&c'` | | `url_decode('%C3%A9')` | `'é'`: the two bytes `C3 A9` are `é` in UTF-8 | | `url_decode('%c3%a9')` | `'é'`: lower-case hexadecimal digits work too | | `url_decode(url_encode('a b/é'))` | `'a b/é'`: it reverses `url_encode` | | `url_decode('Hello')` | `'Hello'`: text without `%` is unchanged | Text that isn't a valid escape is left as it is, rather than causing an error: | Call | Result | | --- | --- | | `url_decode('a+b')` | `'a+b'`: `+` isn't turned into a space | | `url_decode('%zz')` | `'%zz'`: `zz` isn't hexadecimal | | `url_decode('%')` | `'%'`: no digits follow | **`+` stays `+`.** HTML forms and some query strings encode a space as `+` (`application/x-www-form-urlencoded`). `url_decode` doesn't, so for those values replace `+` with a space first: `url_decode(replace(s, '+', ' '))` ([Examples](/develop/docs/dev-database-reference-sql-functions-string-url-decode#examples) shows it). Do the replacement before decoding, so an encoded plus (`%2B`) still decodes to `+`. **Invalid UTF-8 is an error.** `url_decode('%E9')` decodes to the single byte `E9`, which isn't valid UTF-8 on its own (it's `é` in Latin-1), so the query fails with `Failed to decode string "%E9" using URL decoding - decoded value is invalid UTF8`. On a column, one such row stops the whole query, and it returns no rows, including the rows that would have decoded. ## NULLs and edge cases | Input | Result | | --- | --- | | `NULL` | `NULL` | | `''` (empty string) | `''` | | `'a+b'` | `'a+b'` | | `'%zz'` | `'%zz'` | | `'%'` | `'%'` | | `'%C3%A9'` | `'é'` | | `'%E9'` | Error: `Failed to decode string "%E9" using URL decoding - decoded value is invalid UTF8` | | A number, decimal, date, boolean, `BLOB` or list | Error: `No function matches the given name and argument types` | 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-string-url-decode#set-up-the-examples): ``` run("SELECT url_decode(NULL), url_decode(''), url_decode('a+b'), url_decode('%zz'), url_decode('%'), " "url_decode('%C3%A9')") run("SELECT url_decode('%E9')") run("SELECT url_decode(123)") ``` ``` [(None, '', 'a+b', '%zz', '%', 'é')] OSError: Failed to decode string "%E9" using URL decoding - decoded value is invalid UTF8 InternalError: Binder Error: No function matches the given name and argument types 'url_decode(INTEGER_LITERAL)'. You might need to add explicit type casts. ``` ## Examples Each value decoded. Row 3 keeps its `+`: ``` run(f"SELECT id, param, url_decode(value) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 'q', 'café menu'), (2, 'path', '/data/logs/app.log'), (3, 'name', 'Ada+Lovelace'), (4, 'q', 'R&D'), (5, 'q', None)] ``` For form-encoded values, replace `+` with a space before decoding: ``` run(f"SELECT id, url_decode(replace(value, '+', ' ')) FROM {TABLE} WHERE param = 'name' ORDER BY id LIMIT 10") ``` ``` [(3, 'Ada Lovelace')] ``` Find the requests for files under `/data/`, by their decoded path: ``` run(f"SELECT id, url_decode(value) FROM {TABLE} WHERE url_decode(value) LIKE '/data/%' ORDER BY id LIMIT 10") ``` ``` [(2, '/data/logs/app.log')] ``` ## 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 with a constant, but not a function of a column, so `url_decode()` is always computed by the Query Engine: - **In the `SELECT` list,** the scan reads the column and the Query Engine computes `url_decode()` for each row it returns. - **In `WHERE`,** the scan sends the column for every row, and a `FILTER` step in the Query Engine computes `url_decode()` and keeps the rows that match. No rows are skipped in storage. The plan of the last example shows the `FILTER` step above the scan: ``` plan(f"SELECT id FROM {TABLE} WHERE url_decode(value) LIKE '/data/%'") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ prefix(url_decode(value), │ │ '/data/') │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ value │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Decode once, when you write.** If you query the decoded values often, decode them in the application that loads the rows and store the decoded text. Then a comparison of the raw column with a constant is pushed down to VAST DataBase, so rows that don't match never leave storage. - **Decode the constant, not the column.** Once the column holds decoded text, decode the encoded value you search for, not the column. In `query_text = url_decode('caf%C3%A9%20menu')`, `url_decode()` of a constant is computed once, and the comparison of the raw column with the result is pushed down. - **Narrow the rows with a pushed-down condition first.** In `param = 'path' AND url_decode(value) LIKE '/data/%'`, `param = 'path'` is evaluated in VAST DataBase, so `url_decode()` runs only on the rows that pass it. The plan for `param = 'path'` alone has no `FILTER` step: ``` plan(f"SELECT id FROM {TABLE} WHERE param = 'path'") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ``` ## Limitations and errors - **Escapes that don't decode to valid UTF-8 fail.** `%E9` alone fails with `Failed to decode string "%E9" using URL decoding - decoded value is invalid UTF8`, and on a column one such row stops the whole query. Text encoded from Latin-1 or another single-byte character set can't be decoded with `url_decode`. - **`+` isn't a space.** See [What it does](/develop/docs/dev-database-reference-sql-functions-string-url-decode#what-it-does). - **Strings only.** A number, decimal, date, boolean, `BLOB` or list fails with `No function matches the given name and argument types`. ``` run("SELECT url_decode('caf%E9')") run("SELECT url_decode('\\xAA'::BLOB)") ``` ``` OSError: Failed to decode string "caf%E9" using URL decoding - decoded value is invalid UTF8 InternalError: Binder Error: No function matches the given name and argument types 'url_decode(BLOB)'. You might need to add explicit type casts. ``` ## Similar functions | Function | What it does | Result | | --- | --- | --- | | `url_decode` | decodes `%` escapes | `url_decode('a%20b')` gives `'a b'` | | [url_encode](/develop/docs/dev-database-reference-sql-functions-string-url-encode) | percent-encodes a string | `url_encode('a b')` gives `'a%20b'` | | [replace](/develop/docs/dev-database-reference-sql-functions-string-replace) | replaces a substring; use it for `+` before decoding | `url_decode(replace('a+b', '+', ' '))` gives `'a b'` | | `from_base64` ([Binary, hash and UUID functions](/develop/docs/dev-database-reference-sql-functions-binary-hash-uuid-to-base64)) | decodes Base64 text into a `BLOB` | (see its page) | ``` run("SELECT url_decode('a%20b'), url_encode('a b'), url_decode(replace('a+b', '+', ' '))") ``` ``` [('a b', 'a%20b', 'a b')] ``` Clean up: ``` run(f"DROP TABLE IF EXISTS {TABLE}") run(f"DROP SCHEMA IF EXISTS {SCHEMA}") conn.close() ``` ``` [] [] ``` ## See also - [Text basics](/develop/docs/dev-database-reference-sql-functions-string-text-basics): all the text 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. - [url_encode](/develop/docs/dev-database-reference-sql-functions-string-url-encode) and [replace](/develop/docs/dev-database-reference-sql-functions-string-replace).