--- title: "struct_extract" slug: "dev-database-reference-sql-functions-structs-struct-extract" updated: 2026-10-30T18:19:01Z published: 2026-10-30T18:19:01Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-structs-struct-extract" --- > ## 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_extract *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) › Read a field.* Returns the value of one field of a struct, by its name. `s.field`, `s['field']` and `array_extract(s, 'field')` are the same function; on a struct with unnamed fields, `struct_extract` also takes a position. ## Syntax ``` struct_extract(s, 'field') s.field s['field'] array_extract(s, 'field') struct_extract(s, position) ``` | Argument | Type | Description | | --- | --- | --- | | `s` | a struct | The struct to read. | | `field` | a constant `VARCHAR` | The name of the field. The match ignores case. | | `position` | a constant `BIGINT` | On a struct with unnamed fields only, such as one from [row](/develop/docs/dev-database-reference-sql-functions-structs-row): the position of the field, starting at 1. | **Returns:** the type of the field. **Aliases:** `s.field`, `s['field']`, and `array_extract(s, 'field')` when its first argument is a struct. ## 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_extract`. 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` of two fields, `a INTEGER` and `b VARCHAR`. Row 2 has a NULL field, row 3 has two NULL fields, row 4 has a NULL struct, and row 5 has the largest `INTEGER` and an empty string: ``` 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_extract"' 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))") run(f"INSERT INTO {TABLE} VALUES " "(1, {'a': 1, 'b': 'x'}), " "(2, {'a': -7, 'b': NULL}), " "(3, {'a': NULL, 'b': NULL}), " "(4, NULL), " "(5, {'a': 2147483647, 'b': ''})") ``` ``` [] [] [(5,)] ``` ## What it does `struct_extract(s, 'field')` returns the value of the field called `field`, with the field's type: - **Four spellings, one function.** `s.field`, `s['field']`, `struct_extract(s, 'field')` and `array_extract(s, 'field')` return the same values. The dot form also works as `(s).field`, `table.s.field` and `"s"."field"`. - **Names ignore case.** `struct_extract({'a': 1}, 'A')` and `({'A': 1}).a` both return 1. - **Any name works in the string forms,** including names with a space, a dot or a non-ASCII letter: `struct_extract({'a.b': 1}, 'a.b')` is 1. In the dot form, quote such a name: `s."a b"`. - **Nested fields** chain: `s.a.b`, or `struct_extract(struct_extract(s, 'a'), 'b')`. A field that is a list can be indexed: `({'a': [1, 2]}).a[2]` is 2. - **The name must be a constant** and the field must exist; both are checked when the query is planned, before any row is read. - **Unnamed fields are read by position.** On a struct from [row](/develop/docs/dev-database-reference-sql-functions-structs-row), `struct_extract(row(1, 'x'), 2)` and `row(1, 'x')[2]` return `'x'`. For a named struct, use [struct_extract_at](/develop/docs/dev-database-reference-sql-functions-structs-struct-extract-at). ## NULLs and edge cases | Input | Result | | --- | --- | | A NULL struct: `struct_extract(NULL::STRUCT(a BIGINT), 'a')` | `NULL` | | `struct_extract(NULL, 'a')` | `NULL` | | A NULL name: `struct_extract({'a': 1}, NULL)` | `NULL` | | `struct_extract({'a': 1}, 'A')`, `struct_extract({'A': 1}, 'a')` | `1` | | `struct_extract({'a b': 1}, 'a b')`, `struct_extract({'a.b': 1}, 'a.b')`, `struct_extract({'\u00e9': 1}, '\u00e9')` | `1` | | `struct_extract(struct_extract({'a': {'b': 2}}, 'a'), 'b')` | `2` | | `struct_extract(row(1, 'x'), 2)` | `'x'` | | `struct_extract({'a': 1}, 'b')` (a missing field) | Error: `Binder Error: Could not find key "b" in struct`, followed by the names it has | | `struct_extract({'a': 1}, '')` (an empty name) | Error: `Key name for struct_extract needs to be neither NULL nor empty` | 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-extract#set-up-the-examples): ``` run("SELECT struct_extract(NULL::STRUCT(a BIGINT), 'a'), struct_extract(NULL, 'a'), struct_extract({'a': 1}, NULL)") run("SELECT struct_extract({'a': 1}, 'A'), struct_extract({'A': 1}, 'a'), struct_extract({'a b': 1}, 'a b'), " "struct_extract({'a.b': 1}, 'a.b'), struct_extract({'\u00e9': 1}, '\u00e9')") run("SELECT struct_extract(struct_extract({'a': {'b': 2}}, 'a'), 'b'), struct_extract(row(1, 'x'), 2)") run("SELECT struct_extract({'a': 1}, 'b')") run("SELECT struct_extract({'a': 1}, '')") ``` ``` [(None, None, None)] [(1, 1, 1, 1, 1)] [(2, 'x')] InternalError: Binder Error: Could not find key "b" in struct InternalError: Binder Error: Key name for struct_extract needs to be neither NULL nor empty ``` A field read from a NULL struct and a NULL field both give NULL. To tell them apart, test the struct itself: `st IS NULL` is true for row 4 only, while `st.a IS NULL` is true for rows 3 and 4: ``` run(f"SELECT id FROM {TABLE} WHERE st IS NULL ORDER BY id LIMIT 10") run(f"SELECT id FROM {TABLE} WHERE st.a IS NULL ORDER BY id LIMIT 10") ``` ``` [(4,)] [(3,), (4,)] ``` ## Examples Each field, in each spelling: ``` run(f"SELECT id, st.a, st['b'], struct_extract(st, 'a'), array_extract(st, 'b') FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 1, 'x', 1, 'x'), (2, -7, None, -7, None), (3, None, None, None, None), (4, None, None, None, None), (5, 2147483647, '', 2147483647, '')] ``` Keep only the rows whose field `a` is positive: ``` run(f"SELECT id, st.b FROM {TABLE} WHERE st.a > 0 ORDER BY id LIMIT 10") ``` ``` [(1, 'x'), (5, '')] ``` ## 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`): - **In the `SELECT` list,** the scan reads the field the query names, and the Query Engine returns it. - **In `WHERE`, a comparison of a field with a constant is evaluated in VAST DataBase** when it's written `s.field` or `struct_extract(s, 'field')`: `=`, `>` and `BETWEEN` are pushed down, rows that don't match never leave storage, and the plan has no `FILTER` step. - **Other conditions on a field run in a `FILTER` step** in the Query Engine: the bracket form `s['field'] = ...`, `array_extract(s, 'field') = ...`, `IN (...)`, `LIKE`, `IS NULL` and `IS NOT NULL`, and a comparison of the whole struct. The plan of `st.a = 1` has no `FILTER` step; the same condition written with brackets has one: ``` plan(f"SELECT id FROM {TABLE} WHERE st.a = 1") plan(f"SELECT id FROM {TABLE} WHERE st['a'] = 1") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (array_extract(st, 'a') = │ │ 1) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ st.a │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Filter with `s.field` or `struct_extract(s, 'field')`,** not with `s['field']` or `array_extract`: only the first two are pushed down to VAST DataBase. - **Use `BETWEEN` for a range of values.** `st.a BETWEEN 0 AND 2` is pushed down; `st.a IN (1, -7)` runs in a `FILTER` step: ``` plan(f"SELECT id FROM {TABLE} WHERE st.a BETWEEN 0 AND 2") plan(f"SELECT id FROM {TABLE} WHERE st.a IN (1, -7)") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ ((struct_extract(st, 'a') │ │ = 1) OR (struct_extract(st│ │ , 'a') = -7)) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ st.a │ │ id │ │ │ └───────────────────────────┘ ``` - **Select the fields you need, not the whole struct.** `SELECT st.b` reads only that field. The plan lists what the scan reads: ``` plan(f"SELECT st.b FROM {TABLE}") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ struct_extract(st, 'b') │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: st.b │ │ │ └───────────────────────────┘ ``` ## Limitations and errors - **The field must exist.** A missing name fails when the query is planned with `Could not find key "..." in struct`, even if no row would reach it. - **The name must be a constant.** `struct_extract(st, k)` with a column `k`, and `st[k]`, fail with `Key name for struct_extract needs to be a constant string`. - **Names on named structs, positions on unnamed ones.** A position on a named struct fails with `struct_extract with an integer key can only be used on unnamed structs, use a string key instead`; a name on a struct from `row()` fails with `struct_extract with a string key cannot be used on an unnamed struct, use a numeric index instead`. - **A field keeps its type in arithmetic.** `st.a` is an `INTEGER`, so `st.a + 1` on row 5 fails with `Overflow in addition of INT32 (2147483647 + 1)!`. Cast the field to `BIGINT` first. - **The first argument must be a struct.** A string, number, list, map or date fails with `No function matches the given name and argument types`; the error lists `struct_extract(STRUCT, VARCHAR)` and `struct_extract(STRUCT, BIGINT)`. On a list, `array_extract` reads an element by position instead ([list_extract](/develop/docs/dev-database-reference-sql-functions-lists-list-extract)). - **Other SQL dialects' names aren't available.** `get_field` fails with `Scalar Function with name get_field does not exist!`. Use `struct_extract` or `s.field`. ``` run(f"SELECT id, st.c FROM {TABLE} ORDER BY id LIMIT 10") run("SELECT struct_extract({'a': 1}, 1)") run("SELECT struct_extract(row(1, 'x'), 'a')") run(f"SELECT id, st.a + 1 FROM {TABLE} ORDER BY id LIMIT 10") run(f"SELECT id, CAST(st.a AS BIGINT) + 1 FROM {TABLE} ORDER BY id LIMIT 10") run("SELECT get_field({'a': 1}, 'a')") ``` ``` InternalError: Binder Error: Could not find key "c" in struct InternalError: Binder Error: struct_extract with an integer key can only be used on unnamed structs, use a string key instead InternalError: Binder Error: struct_extract with a string key cannot be used on an unnamed struct, use a numeric index instead OSError: Overflow in addition of INT32 (2147483647 + 1)! [(1, 2), (2, -6), (3, None), (4, None), (5, 2147483648)] InternalError: Catalog Error: Scalar Function with name get_field does not exist! ``` ## Similar functions | Function | Reads | Result for `{'a': 1, 'b': 'x'}` | | --- | --- | --- | | `struct_extract(s, 'b')`, `s.b`, `s['b']`, `array_extract(s, 'b')` | a field by name | `'x'` | | [struct_extract_at](/develop/docs/dev-database-reference-sql-functions-structs-struct-extract-at)`(s, 2)` | a field by position | `'x'` | | `json_extract_string(to_json(s), '$.b')` | a field of the struct's JSON text | `'x'` | ``` run("SELECT struct_extract({'a': 1, 'b': 'x'}, 'b'), {'a': 1, 'b': 'x'}.b, {'a': 1, 'b': 'x'}['b'], " "array_extract({'a': 1, 'b': 'x'}, 'b'), struct_extract_at({'a': 1, 'b': 'x'}, 2), " "json_extract_string(to_json({'a': 1, 'b': 'x'}), '$.b')") ``` ``` [('x', 'x', 'x', 'x', 'x', 'x')] ``` Clean up: ``` 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_pack](/develop/docs/dev-database-reference-sql-functions-structs-struct-pack): build a struct. - [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.