"SELECT [value]" vs "SELECT [value] FROM [table] LIMIT 1" in MySQL - mysql

Question

Which query is better ?
SELECT true;
SELECT true FROM users LIMIT 1;
In terms of:
Best practice
Performance

Answer 1

The first query has less overhead because it doesn't reference any tables.
mysql> explain select true\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: NULL
partitions: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
filtered: NULL
Extra: No tables used
Whereas the second query does reference a table, which means it has to spend time:
Checking that the table exists and if the query references any columns, check that the columns exist.
Checking that your user has privileges to read that table.
Acquiring a metadata lock, so no one does any DDL or LOCK TABLES while your query is reading it.
Starting to do an index-scan, even though it will be cut short by the LIMIT.
Here's the explain for the second query for comparison:
mysql> explain select true from mysql.user limit 1\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: user
partitions: NULL
type: index
possible_keys: NULL
key: PRIMARY
key_len: 276
ref: NULL
rows: 8
filtered: 100.00
Extra: Using index

Answer 2

First query will one row with value true.
Second query will return all the rows from users table but true as only value.
So you if you need one row user first query. But if you need all the rows with same value then use second one.

Answer 3

In either case, it is obvious you want the value of TRUE :) With this intention, the "SELECT TRUE" is the most efficient as it won't cause MySQL to go further looking for users table, no matter how many rows in it, and then go even further with "LIMIT 1" if there are rows!
By the term BEST PRACTICE, I am not sure what you meant here, because, from my point of view, this doesn't even require a PRACTICE, let alone BEST, as I fail to see any real life application of this approach.