sql: "char" is supposed to truncate long values

[rafiss]

In postgres:

rafiss@127:postgres> create table example (a "char");
CREATE TABLE

rafiss@127:postgres> insert into example values ('abc');
INSERT 0 1

rafiss@127:postgres> select a from example;
+-----+
| a   |
|-----|
| a   |
+-----+
SELECT 1

In cockroachdb

root@:26257/defaultdb> create table example (a "char");
CREATE TABLE

root@:26257/defaultdb>  insert into example values ('abc');
INSERT 1

root@:26257/defaultdb>  select a from example;
   a
-------
  abc
(1 row)

Note that this is DIFFERENT than CHAR -- CHAR will only truncate long values if the extra characters are whitespace, and cockroachdb already implements this correctly.

Epic CRDB-7217

[Polokghosh53]

I am a beginner to open source contribution. How should I get started with this issue?

[rafiss]

The change here will be fairly similar to #50492 -- that PR truncates whitespace for CHAR types.

The steps you'll need are:

1. update casting logic in pkg/sql/sem/tree/casts.go -- the AdjustValueToType function should have a new case for if typ.Oid() == oid.T_char and perform truncation.
2. update constant resolution logic in pkg/sql/sem/tree/constant.go -- the ResolveAsType should have two cases for if typ.Oid() == oid.T_char
3. Add tests (you could also start with this step first and work backwards to the code change) -- add tests in pkg/sql/logictest/testdata/logic_test/collatedstring and pkg/sql/sem/tree/testdata/eval/cast similar to the ones in the PR i linked.