Support ascii function

[yikf]

Description

Ascii function is very useful, some databases support it, for example:

• Mysql: https://dev.mysql.com/doc/refman/5.6/en/string-functions.html#function_ascii
• PostgresSQL: https://www.postgresql.org/docs/14/functions-string.html
• SparkSql: https://spark.apache.org/docs/latest/api/sql/index.html#ascii

It has the following behavior:
Returns the numeric value of the leftmost character of the string str. Returns 0 if str is the empty string. Returns NULL if str is NULL. ASCII works for 8-bit characters.

Related issues, pull requests, and links

Documentation

( ) No documentation is needed.
( ) Sufficient documentation is included in this PR.
( ) Documentation PR is available with #prnumber.
( ) Documentation issue #issuenumber is filed, and can be handled later.

Release notes

( ) No release notes entries required.
( ) Release notes entries required with the following suggested text:

# Section
* Fix some things. ({issue}`issuenumber`)

[yikf]

@findepi Can you help take a look, thanks

[findepi]

we already have codepoint https://trino.io/docs/current/functions/string.html#codepoint for this purpose
i think we shouldn't add this one.

[yikf]

we already have codepoint https://trino.io/docs/current/functions/string.html#codepoint for this purpose i think we shouldn't add this one.

Yes, i noticed that before this, but i think the two are different, and this is inconsistent with some standard function behavior.

For example, if i want to calculate the ASCII of a varchar column, codepoint maybe thrown a Input string must be a single character string exception, but function ascii evaluate correctly, at this point, i think we should keep consistent with MySQL, PostgresSQL, SparkSQL etc.

[yikf]

we already have codepoint https://trino.io/docs/current/functions/string.html#codepoint for this purpose i think we shouldn't add this one.

Yes, i noticed that before this, but i think the two are different, and this is inconsistent with some standard function behavior.

For example, if i want to calculate the ASCII of a varchar column, codepoint maybe thrown a Input string must be a single character string exception, but function ascii evaluate correctly, at this point, i think we should keep consistent with MySQL, PostgresSQL, SparkSQL etc.

ping @findepi

[findepi]

For example, if i want to calculate the ASCII of a varchar column, codepoint maybe thrown a Input string must be a single character string exception,

You can use substring for this.

but function ascii evaluate correctly, at this point, i think we should keep consistent with MySQL, PostgresSQL, SparkSQL etc.

it's quite unusual for a function to operate on the first character of the input and ignore the rest.
it should be reflected in the function name

i understand we could want to have eg PostgreSQL-compatible ascii function, but proposed semantics aren't good for a global ascii function, so at minimum, we need function namespaces first (#8).