--- title: "substring_grapheme" slug: "dev-database-reference-sql-functions-string-substring-grapheme" updated: 2026-10-30T18:18:56Z published: 2026-10-30T18:18:56Z canonical: "docs.vastdata.com/dev-database-reference-sql-functions-string-substring-grapheme" --- > ## 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. # substring_grapheme *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) › Parts of a string.* Returns part of a string, counting positions and length in characters as a reader sees them (grapheme clusters): `length` of them from position `start`, or everything from `start` to the end. There are no aliases. ## Syntax ``` substring_grapheme(s, start) substring_grapheme(s, start, length) ``` | Argument | Type | Description | | --- | --- | --- | | `s` | `VARCHAR` | The string to take part of. | | `start` | `BIGINT` (any integer) | Where to start, in grapheme clusters. Positions count from 1; a negative `start` counts from the end, `-1` being the last grapheme cluster. | | `length` | `BIGINT` (any integer) | How many grapheme clusters to take. Leave it out to take everything to the end. | **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_substring_grapheme`. 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 posts. Two posts start with a flag, one ends in a combining accent; 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_substring_grapheme"' 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}.posts" run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}") run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, author VARCHAR, body VARCHAR)") run(f"INSERT INTO {TABLE} VALUES " "(1, 'E\u0301mile', '\U0001F1EB\U0001F1F7 Paris in spring'), " "(2, 'Jose\u0301', 'Lunch at the cafe\u0301'), " "(3, 'Grace', '\U0001F1FA\U0001F1F8 New York'), " "(4, NULL, NULL)") ``` ``` [] [] [(4,)] ``` ## What it does `substring_grapheme` works like [substring](/develop/docs/dev-database-reference-sql-functions-string-substring), but counts grapheme clusters instead of code points. A **code point** is one Unicode value: the unit [length](/develop/docs/dev-database-reference-sql-functions-string-length) and [substring](/develop/docs/dev-database-reference-sql-functions-string-substring) count. A **grapheme cluster** is what a reader sees as one character, and it can be several code points: a letter followed by a combining accent (`e` and U+0301, displayed as é) is one grapheme cluster of two code points, and so is a flag emoji such as 🇫🇷. An emoji made of several emoji joined by zero-width joiners, such as a family, is also one grapheme cluster ([length_grapheme](/develop/docs/dev-database-reference-sql-functions-string-length-grapheme) counts it as 1). Counting grapheme clusters keeps positions where a reader expects them, and never returns half a character: | Call | Result | With `substring` instead | | --- | --- | --- | | `substring_grapheme('cafe\u0301', 3, 2)` | `'fe\u0301'` (fé) | `'fe'`: the 2 code points are `f` and `e`, without the accent | | `substring_grapheme('🇫🇷🇩🇪', 2, 1)` | `'🇩🇪'` | half of the French flag: each flag is 2 code points | | `substring_grapheme('e\u0301 🇫🇷 ẞ', 2, 2)` | `' 🇫🇷'` (a space and the flag) | `substring(…, 2, 3)` returns the accent alone, a space and half the flag | Both columns, as the cluster returns them, with the number of code points in some of the results. They use the table and helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-string-substring-grapheme#set-up-the-examples): ``` run("SELECT substring_grapheme('cafe\u0301', 3, 2), substring('cafe\u0301', 3, 2), " "substring_grapheme('\U0001F1EB\U0001F1F7\U0001F1E9\U0001F1EA', 2, 1), " "length(substring('\U0001F1EB\U0001F1F7\U0001F1E9\U0001F1EA', 2, 1))") run("SELECT substring_grapheme('e\u0301 \U0001F1EB\U0001F1F7 \u1E9E', 2, 2), " "length(substring('e\u0301 \U0001F1EB\U0001F1F7 \u1E9E', 2, 3))") ``` ``` [('fé', 'fe', '🇩🇪', 1)] [(' 🇫🇷', 3)] ``` The positions follow the same rules as `substring`: | `start` | Example | Result | | --- | --- | --- | | Positive: counts from 1 | `substring_grapheme('abc', 2)` | `'bc'` | | `0`: an empty position before the first grapheme cluster, which counts towards `length` | `substring_grapheme('abc', 0, 2)` | `'a'` | | Negative: counts from the end | `substring_grapheme('abc', -1, 1)` | `'c'` | ## NULLs and edge cases | Input | Result | | --- | --- | | `s` is NULL | `NULL` | | `start` is NULL | `NULL` | | `''` (empty string) | `''` | | `substring_grapheme('abc', 2)` | `'bc'` | | `substring_grapheme('abc', 0, 2)` | `'a'` | | `substring_grapheme('abc', -1, 1)` | `'c'` | | `substring_grapheme('cafe\u0301', 3, 2)` | `'fe\u0301'` (fé, three code points) | | `substring_grapheme('🇫🇷🇩🇪', 2, 1)` | `'🇩🇪'` | | `length` is NULL: `substring_grapheme('abc', 1, NULL)` | `NULL` | | A length of 0: `substring_grapheme('abc', 2, 0)` | `''` | | A negative length: `substring_grapheme('abc', 2, -1)` | `'a'` (the grapheme cluster before `start`, as with `substring`) | | A start past the end: `substring_grapheme('abc', 10)` | `''` | | `s` a number, 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-substring-grapheme#set-up-the-examples): ``` run("SELECT substring_grapheme(NULL, 2, 2), substring_grapheme('abc', NULL, 1), substring_grapheme('', 2, 2), " "substring_grapheme('abc', 2), substring_grapheme('abc', 0, 2), substring_grapheme('abc', -1, 1)") run("SELECT substring_grapheme('cafe\u0301', 3, 2), length(substring_grapheme('cafe\u0301', 3, 2)), " "substring_grapheme('\U0001F1EB\U0001F1F7\U0001F1E9\U0001F1EA', 2, 1)") run("SELECT substring_grapheme('abc', 1, NULL), substring_grapheme('abc', 2, 0), substring_grapheme('abc', 2, -1), " "substring_grapheme('abc', 10)") run("SELECT substring_grapheme(123, 2, 2)") ``` ``` [(None, None, '', 'bc', 'a', 'c')] [('fé', 3, '🇩🇪')] [(None, '', 'a', '')] InternalError: Binder Error: No function matches the given name and argument types 'substring_grapheme(INTEGER_LITERAL, INTEGER_LITERAL, INTEGER_LITERAL)'. You might need to add explicit type casts. ``` ## Examples Each post without its first two characters, counted both ways. The flag is one grapheme cluster but two code points, so `substring` from position 3 keeps the space after it: ``` run(f"SELECT id, substring_grapheme(body, 3), substring(body, 3) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 'Paris in spring', ' Paris in spring'), (2, 'nch at the café', 'nch at the café'), (3, 'New York', ' New York'), (4, None, None)] ``` The last four characters of each post, with a negative start. For `café`, `substring` counts the accent as a character and returns `afé`: ``` run(f"SELECT id, substring_grapheme(body, -4), substring(body, -4) FROM {TABLE} ORDER BY id LIMIT 10") ``` ``` [(1, 'ring', 'ring'), (2, 'café', 'afé'), (3, 'York', 'York'), (4, None, None)] ``` Find the posts whose third to seventh grapheme clusters are `Paris`: ``` run(f"SELECT id FROM {TABLE} WHERE substring_grapheme(body, 3, 5) = 'Paris' ORDER BY id LIMIT 10") ``` ``` [(1,)] ``` ## 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 `substring_grapheme()` is always computed by the Query Engine: - **In the `SELECT` list,** the scan reads the column and the Query Engine computes `substring_grapheme()` for each row it returns. - **In `WHERE`,** the scan sends the column for every row, and a `FILTER` step in the Query Engine computes `substring_grapheme()` and keeps the rows that match. No rows are skipped in storage. The plan of the `WHERE` example shows the `FILTER` step above the scan: ``` plan(f"SELECT id FROM {TABLE} WHERE substring_grapheme(body, 3, 5) = 'Paris'") ``` ``` ┌───────────────────────────┐ │ PROJECTION │ │ ──────────────────── │ │ #1 │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ FILTER │ │ ──────────────────── │ │ (substring_grapheme(body, │ │ 3, 5) = 'Paris') │ │ │ └─────────────┬─────────────┘ ┌─────────────┴─────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ │ │ Projections: │ │ body │ │ id │ │ │ └───────────────────────────┘ ``` ## Make it faster - **Use `substring` when the text is ASCII.** For text with no accents, emoji or other multi-code-point characters, such as codes and identifiers, a code point is a grapheme cluster, and [substring](/develop/docs/dev-database-reference-sql-functions-string-substring) gives the same result. - **Store the part you filter on.** If you often filter on one part of a value, keep it in its own column when you write the rows: a comparison of a raw column with a constant is pushed down to VAST DataBase, so rows that don't match never leave storage. The plan has no `FILTER` step: ``` plan(f"SELECT id FROM {TABLE} WHERE author = 'Grace'") ``` ``` ┌───────────────────────────┐ │ VAST_SCAN │ │ ──────────────────── │ │ Function: VAST_SCAN │ │ Projections: id │ │ │ └───────────────────────────┘ ``` - **Combine it with a pushed-down condition.** In `id > 100 AND substring_grapheme(body, 3, 5) = 'Paris'`, `id > 100` is still evaluated in VAST DataBase, so `substring_grapheme()` runs only on the rows that pass it. ## Limitations and errors - **`VARCHAR` only.** A number, a date, a boolean, a `BLOB` or a list fails with `No function matches the given name and argument types`. Cast a value to text first: `substring_grapheme(CAST(n AS VARCHAR), 2, 2)`. - **Positions and lengths must be integers.** A `DECIMAL` such as `1.5` fails with `No function matches the given name and argument types 'substring_grapheme(STRING_LITERAL, DECIMAL(2,1))'`; it isn't converted for you. Cast it to an integer first: `substring_grapheme(s, CAST(x AS BIGINT))`. The cast rounds, it doesn't truncate. - **No `FROM … FOR` form and no slices.** `substring_grapheme('abc' FROM 2 FOR 1)` fails with `Parser Error: syntax error at or near "FROM"`. Pass the start and length as arguments: `substring_grapheme('abc', 2, 1)`. The slice forms `s[start:end]` and `s[i]` count code points, like [substring](/develop/docs/dev-database-reference-sql-functions-string-substring). - **Grapheme clusters, not normalised text.** `substring_grapheme` keeps the code points it finds: a decomposed é comes back as two code points, as the `length()` in [NULLs and edge cases](/develop/docs/dev-database-reference-sql-functions-string-substring-grapheme#nulls-and-edge-cases) shows. [nfc_normalize](/develop/docs/dev-database-reference-sql-functions-string-nfc-normalize) turns it into the one-code-point `é`. ``` run("SELECT substring_grapheme(123, 2, 2)") run("SELECT substring_grapheme(CAST(12345 AS VARCHAR), 2, 2)") ``` ``` InternalError: Binder Error: No function matches the given name and argument types 'substring_grapheme(INTEGER_LITERAL, INTEGER_LITERAL, INTEGER_LITERAL)'. You might need to add explicit type casts. [('23',)] ``` ## Similar functions | Function | Takes | Result for `'e\u0301x'` (éx), position 1, length 1 | | --- | --- | --- | | `substring_grapheme(s, start, length)` | grapheme clusters by position | `'e\u0301'` (é) | | [substring](/develop/docs/dev-database-reference-sql-functions-string-substring) | code points by position | `'e'` | | [left_grapheme](/develop/docs/dev-database-reference-sql-functions-string-left-grapheme) `(s, 1)` | the first `n` grapheme clusters | `'e\u0301'` (é) | | [left](/develop/docs/dev-database-reference-sql-functions-string-left) `(s, 1)` | the first `n` code points | `'e'` | | [right_grapheme](/develop/docs/dev-database-reference-sql-functions-string-right-grapheme) | the last `n` grapheme clusters | | | [length_grapheme](/develop/docs/dev-database-reference-sql-functions-string-length-grapheme) | counts the grapheme clusters | | ``` run("SELECT substring_grapheme('e\u0301x', 1, 1), substring('e\u0301x', 1, 1), left_grapheme('e\u0301x', 1), " "left('e\u0301x', 1)") ``` ``` [('é', 'e', 'é', 'e')] ``` 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. - [substring](/develop/docs/dev-database-reference-sql-functions-string-substring), [left_grapheme](/develop/docs/dev-database-reference-sql-functions-string-left-grapheme), [right_grapheme](/develop/docs/dev-database-reference-sql-functions-string-right-grapheme) and [length_grapheme](/develop/docs/dev-database-reference-sql-functions-string-length-grapheme). - [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.