reverse

Applies to VAST Cluster 5.5.

Functions › Text basics › Changing text. Returns a string with its characters in reverse order. A letter with a combining accent and a flag emoji stay whole. There are no aliases.

Syntax

reverse(s)

Returns: VARCHAR.

Aliases: none.

Set up the examples

The examples connect with the ADBC driver, as in VAST Query Engine (ADBC), and work in a schema called docs_sql_fn_reverse. Set QE_ENDPOINT, AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY and VASTDB_BUCKET (SQL reference), 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 words. été is written with combining accents, and 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_reverse"'

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, "<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}.words"
run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}")
run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, word VARCHAR)")
run(f"INSERT INTO {TABLE} VALUES "
    "(1, 'level'), "
    "(2, 'Hello'), "
    "(3, 'e\u0301te\u0301'), "
    "(4, 'kayak'), "
    "(5, NULL)")

[]
[]
[(5,)]

What it does

reverse reverses the order of the characters as a reader sees them (grapheme clusters), not of the code points they're made of:

This is different from left, right and substring, which count code points and can split a letter from its accent.

NULLs and edge cases

The same cases, as the cluster returns them. They use the table and helpers from Set up the examples. strpos() of the accent shows it is still the last code point, after the e:

run("SELECT reverse(NULL), reverse(''), reverse('abc'), reverse('e\u0301x'), strpos(reverse('e\u0301x'), '\u0301')")
run("SELECT reverse('\U0001F1EB\U0001F1F7'), reverse('\U0001F1EB\U0001F1F7') = '\U0001F1EB\U0001F1F7'")
run("SELECT reverse(123)")
run("SELECT reverse(['a', 'b'])")

[(None, '', 'cba', 'xé', 3)]
[('🇫🇷', True)]
InternalError: Binder Error: No function matches the given name and argument types 'reverse(INTEGER_LITERAL)'. You might need to add explicit type casts.
InternalError: Binder Error: No function matches the given name and argument types 'reverse(VARCHAR[])'. You might need to add explicit type casts.

Examples

Each word reversed:

run(f"SELECT id, word, reverse(word) FROM {TABLE} ORDER BY id LIMIT 10")

[(1, 'level', 'level'), (2, 'Hello', 'olleH'), (3, 'été', 'été'), (4, 'kayak', 'kayak'), (5, None, None)]

The palindromes: the words that read the same backwards. été is one, because its accents stay after their letters when it's reversed:

run(f"SELECT id, word FROM {TABLE} WHERE word = reverse(word) ORDER BY id LIMIT 10")

[(1, 'level'), (3, 'été'), (4, 'kayak')]

Find a word from its reversed spelling, with reverse() on the constant:

run(f"SELECT id, word FROM {TABLE} WHERE word = reverse('olleH') ORDER BY id LIMIT 10")

[(2, 'Hello')]

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 reverse() of a column is always computed by the Query Engine:

• In the SELECT list, the scan reads the column and the Query Engine computes reverse() for each row it returns.
• In WHERE, the scan sends the column for every row, and a FILTER step in the Query Engine computes reverse() and keeps the rows that match. No rows are skipped in storage.

The plan of the palindrome example shows the FILTER step above the scan:

plan(f"SELECT id FROM {TABLE} WHERE word = reverse(word)")

┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│   (word = reverse(word))  │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│                           │
│        Projections:       │
│            word           │
│             id            │
│                           │
└───────────────────────────┘

Make it faster

• Reverse the constant, not the column. reverse(word) = 'olleH' computes reverse() for every row in a FILTER step. word = reverse('olleH') asks the same question, but reverse() runs once, on the constant, and the comparison of the raw column with the result is pushed down to VAST DataBase. The plan has no FILTER step:

plan(f"SELECT id FROM {TABLE} WHERE word = reverse('olleH')")

┌───────────────────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│      Projections: id      │
│                           │
└───────────────────────────┘

• Store the result if you filter on it often. A test that needs reverse() of the column, such as the palindrome test, can't be pushed down. Compute it when you write the rows and keep it in its own column, such as is_palindrome: is_palindrome = true is pushed down.
• Combine it with a pushed-down condition. In id > 100 AND word = reverse(word), id > 100 is still evaluated in VAST DataBase, so reverse() runs only on the rows that pass it.

Limitations and errors

• VARCHAR only. A number, a date, a boolean or a BLOB fails with No function matches the given name and argument types. Cast a value to text first: reverse(CAST(n AS VARCHAR)).
• Not for lists. reverse(['a', 'b']) fails with No function matches the given name and argument types 'reverse(VARCHAR[])'. Use list_reverse.
• No strrev. strrev('abc') fails with Scalar Function with name strrev does not exist!. Use reverse.

run("SELECT reverse(123)")
run("SELECT reverse(CAST(123 AS VARCHAR)), list_reverse(['a', 'b'])")

InternalError: Binder Error: No function matches the given name and argument types 'reverse(INTEGER_LITERAL)'. You might need to add explicit type casts.
[('321', ['b', 'a'])]

Similar functions

run("SELECT reverse('e\u0301x'), list_reverse([1, 2, 3])")

[('xé', [3, 2, 1])]

Clean up:

run(f"DROP TABLE IF EXISTS {TABLE}")
run(f"DROP SCHEMA IF EXISTS {SCHEMA}")
conn.close()

[]
[]

See also

• Text basics: all the text functions.
• list_reverse: reverse a list.
• Where filters run and pushdowns.
• EXPLAIN: how to read a plan.