--- title: "map_from_entries" slug: "dev-database-reference-sql-functions-maps-map-from-entries" updated: 2026-10-30T18:18:46Z published: 2026-10-30T18:18:46Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-maps-map-from-entries" --- > ## 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. # map_from_entries *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) › Build a map.* Builds a map from a list of structs with two fields: the first field of each struct is a key, the second its value. ## Syntax ``` map_from_entries(entries) ``` | Argument | Type | Description | | --- | --- | --- | | `entries` | a list of structs with exactly two fields | The pairs. The first field is the key and the second the value, whatever the fields are called. | **Returns:** `MAP(key type, value type)`, the types of the two fields. ## 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_maps_map_from_entries`. 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 articles. `counts` holds the word count of each translation as a list of structs; `langs` and `words` hold the same kind of data as two lists. Article 2 has a translation with no word count yet, article 3 has no translations, and the last row has 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_maps_map_from_entries"' 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}.articles" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} " "(id BIGINT, counts STRUCT(lang VARCHAR, words BIGINT)[], langs VARCHAR[], words BIGINT[])") run(f"INSERT INTO {TABLE} VALUES " "(1, [{'lang': 'en', 'words': 120}, {'lang': 'de', 'words': 135}], ['en', 'de'], [120, 135]), " "(2, [{'lang': 'fr', 'words': NULL}], ['fr', 'es'], [NULL, 140]), " "(3, [], [], []), " "(4, NULL, NULL, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does Each struct in the list becomes one pair of the map, in the order of the list. Only the position of the fields counts: `{'k': 'a', 'v': 1}`, `{'key': 'a', 'value': 1}` and the unnamed `('a', 1)` all give the key `'a'` with the value `1`, and `{'v': 1, 'k': 'a'}` gives the key `1` with the value `'a'`. `map_from_entries` is the inverse of [map_entries](/develop/docs/dev-database-reference-sql-functions-maps-map-entries): `map_from_entries(map_entries(m))` is `m`. Between the two, the [list functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs) can filter or change the pairs. Python receives a map as a list of `(key, value)` tuples. ## NULLs and edge cases | Input | Result | | --- | --- | | `map_from_entries(NULL)` | `NULL` | | `map_from_entries([{'k': 'a', 'v': NULL}])` | `[('a', None)]`: NULL values are allowed | | `map_from_entries([]::STRUCT(k VARCHAR, v BIGINT)[])` | an empty map, `[]` | | `map_from_entries([('a', 1), ('b', 2)])` | `[('a', 1), ('b', 2)]` | | `map_from_entries([{'v': 1, 'k': 'a'}])` | `[(1, 'a')]`: the first field is the key | | `map_from_entries([{'k': NULL, 'v': 1}])`, a NULL struct in the list | Error: `Map keys can not be NULL.` | | `map_from_entries([{'k': 'a', 'v': 1}, {'k': 'a', 'v': 2}])` | Error: `Map keys must be unique.` | | `map_from_entries([])` (an empty list with no type) | Error: `The elements of the list must be structs` | | `map_from_entries([{'k': 'a'}])` or three fields | Error: `The provided struct type should only contain 2 fields, a key and a value` | | `map_from_entries('abc')` or a struct or a map | Error: `The provided argument is not a list of structs` | 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-maps-map-from-entries#set-up-the-examples): ``` run("SELECT map_from_entries(NULL), map_from_entries([{'k': 'a', 'v': NULL}]), " "map_from_entries([]::STRUCT(k VARCHAR, v BIGINT)[])") run("SELECT map_from_entries([('a', 1), ('b', 2)]), map_from_entries([{'v': 1, 'k': 'a'}])") run("SELECT map_from_entries([{'k': NULL, 'v': 1}])") run("SELECT map_from_entries([{'k': 'a', 'v': 1}, {'k': 'a', 'v': 2}])") run("SELECT map_from_entries([])") run("SELECT map_from_entries([{'k': 'a'}])") run("SELECT map_from_entries('abc')") ``` ``` [(None, [('a', None)], [])] [([('a', 1), ('b', 2)], [(1, 'a')])] OSError: Map keys can not be NULL. OSError: Map keys must be unique. InternalError: Invalid Input Error: The elements of the list must be structs InternalError: Invalid Input Error: The provided struct type should only contain 2 fields, a key and a value InternalError: Invalid Input Error: The provided argument is not a list of structs ``` ## Examples Build a map per article from its list of structs. The NULL list of article 4 gives NULL: ``` run(f"SELECT id, map_from_entries(counts) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, [('en', 120), ('de', 135)]), (2, [('fr', None)]), (3, []), (4, None)] ``` Build the structs in the query. [list_zip](/develop/docs/dev-database-reference-sql-functions-lists-list-zip) pairs two lists; [list_transform](/develop/docs/dev-database-reference-sql-functions-lists-list-transform) builds a pair from each element of one list, here each language with its length. `list_zip` treats NULL lists as empty, so article 4 gets an empty map: ``` run(f"SELECT id, map_from_entries(list_zip(langs, words)) FROM {TABLE} ORDER BY id LIMIT 10") run(f"SELECT id, map_from_entries(list_transform(langs, x -> {{'k': x, 'v': length(x)}})) " f"FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, [('en', 120), ('de', 135)]), (2, [('fr', None), ('es', 140)]), (3, []), (4, [])] [(1, [('en', 2), ('de', 2)]), (2, [('fr', 2), ('es', 2)]), (3, []), (4, None)] ``` Change a map through its entries: keep some pairs, change the values, or change the keys: ``` run("SELECT map_from_entries(list_filter(map_entries(MAP {'a': 1, 'b': 2}), e -> e.value > 1))") run("SELECT map_from_entries(list_transform(map_entries(MAP {'a': 1}), " "e -> {'key': e.key, 'value': e.value * 10}))") run("SELECT map_from_entries(list_transform(map_entries(MAP {'a': 1}), " "e -> {'key': upper(e.key), 'value': e.value}))") ``` ``` [([('b', 2)],)] [([('a', 10)],)] [([('A', 1)],)] ``` Parse a string of pairs into a map: ``` run("SELECT map_from_entries(list_transform(string_split('a:1,b:2', ','), " "x -> {'key': split_part(x, ':', 1), 'value': split_part(x, ':', 2)}))") ``` ``` [([('a', '1'), ('b', '2')],)] ``` ## 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`). `map_from_entries()` is always computed by the Query Engine: - **In the `SELECT` list,** the scan sends the list column, and the Query Engine builds the map for each row it returns. - **In `WHERE`,** a `FILTER` step in the Query Engine builds the map and tests it for every row the scan sends. No rows are skipped in storage. The plan of a lookup in `WHERE` shows the `FILTER` step above the scan: ``` plan(f"SELECT id FROM {TABLE} WHERE map_from_entries(counts)['de'] = 135") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (map_extract_value │ │ (map_from_entries(counts),│ │ 'de') = 135) │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ counts │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Pair a map condition with a pushed-down one.** A comparison of a plain column with a constant, such as `id > 1`, is evaluated in VAST DataBase, so the rows it rules out never reach the `FILTER` step. - **Keep a value you filter on in its own column.** If queries often test one key, store its value in a column when you write the rows; a comparison on that column is evaluated in VAST DataBase. ``` run(f"SELECT id, map_from_entries(counts)['de'] FROM {TABLE} WHERE id > 1 ORDER BY id LIMIT 10") ``` ``` [(2, None), (3, None), (4, None)] ``` ## Limitations and errors - **One bad row fails the whole query.** If any row's list has a duplicate key or a NULL key, the query stops with `Map keys must be unique.` or `Map keys can not be NULL.`, even for rows that are fine. Leave such rows out with `WHERE`, or remove the duplicates with [list_filter](/develop/docs/dev-database-reference-sql-functions-lists-list-filter) first. - **An empty list needs a type.** `[]` alone fails; cast it, as in `[]::STRUCT(k VARCHAR, v BIGINT)[]`. A list column always has a type. - **No aggregate builds the list of entries.** `map_from_entries(list(...))` over a table fails, because `list()` isn't available ([aggregate functions](/develop/docs/dev-database-reference-sql-functions-aggregate)). - **A map can't be stored, but its entries can.** Store the pairs in a list-of-structs column and build the map when you read it, as on this page. - **Not available:** `map_filter`, `transform_keys`, `transform_values`, `map_delete` and `str_to_map`. The examples above show how to do each with `map_from_entries`. ``` run(f"SELECT map_from_entries(list({{'k': id, 'v': id}})) FROM {TABLE}") run("SELECT map_from_entries(list_filter([{'k': 'a', 'v': 1}, {'k': 'a', 'v': 2}], e -> e.v > 1))") run("SELECT map_filter(MAP {'a': 1, 'b': 2}, (k, v) -> v > 1)") ``` ``` InternalError: The operator 'HASH_GROUP_BY' type=9 is currently not supported. [([('a', 2)],)] InternalError: Catalog Error: Scalar Function with name map_filter does not exist! ``` ## Similar functions | Function | Builds a map from | Example | | --- | --- | --- | | `map_from_entries` | a list of two-field structs | `map_from_entries([{'k': 'a', 'v': 1}, {'k': 'b', 'v': 2}])` | | [map](/develop/docs/dev-database-reference-sql-functions-maps-map) | a list of keys and a list of values | `map(['a', 'b'], [1, 2])` | | [map_concat](/develop/docs/dev-database-reference-sql-functions-maps-map-concat) | two or more maps | `map_concat(MAP {'a': 1}, MAP {'b': 2})` | | [map_entries](/develop/docs/dev-database-reference-sql-functions-maps-map-entries) | (the inverse: a map's pairs as a list of structs) | `map_entries(MAP {'a': 1, 'b': 2})` | ``` run("SELECT map_from_entries([{'k': 'a', 'v': 1}, {'k': 'b', 'v': 2}]), map(['a', 'b'], [1, 2]), " "map_concat(MAP {'a': 1}, MAP {'b': 2}), map_from_entries(map_entries(MAP {'a': 1, 'b': 2}))") ``` ``` [([('a', 1), ('b', 2)], [('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 - [List, struct and map functions](/develop/docs/dev-database-reference-sql-functions-lists-and-structs): all the map functions. - [map_entries](/develop/docs/dev-database-reference-sql-functions-maps-map-entries): a map's pairs as a list of structs. - [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.