MySQL support two categories of variables, user variables which are
prefixed with the @character and session/global SERVER variables which are
prefixed with @@.
You can also access them via:
Select @@session.session_var;
Select @@global.global_var;
select @@session_or_global_var;
for example:
mysql> select @@warp_adjust_table_stats_for_joins;
+-------------------------------------+
| @@warp_adjust_table_stats_for_joins |
+-------------------------------------+
| 1 |
+-------------------------------------+
1 row in set (0.00 sec)
mysql> set warp_adjust_table_stats_for_joins= false;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@warp_adjust_table_stats_for_joins;
+-------------------------------------+
| @@warp_adjust_table_stats_for_joins |
+-------------------------------------+
| 0 |
+-------------------------------------+
1 row in set (0.00 sec)
mysql> select @@session.warp_adjust_table_stats_for_joins;
+---------------------------------------------+
| @@session.warp_adjust_table_stats_for_joins |
+---------------------------------------------+
| 0 |
+---------------------------------------------+
1 row in set (0.00 sec)
mysql> select @@global.warp_adjust_table_stats_for_joins;
+--------------------------------------------+
| @@global.warp_adjust_table_stats_for_joins |
+--------------------------------------------+
| 1 |
+--------------------------------------------+
1 row in set (0.00 sec)
On Tue, Mar 22, 2022 at 5:02 PM Julian Hyde <jhyde.apa...@gmail.com> wrote:
> The ‘@@‘ prefix is not standard SQL, and Calcite does not support it.
>
> Can you do some research to find out how MySQL handles it. Is it
> considered to be part of the variable name? Or is it a prefix (like $ in
> bash) that means ‘what comes next is a variable’? In other words, does the
> parser say there is a reference to a variable called
> '@@character_set_server’ or a variable called ‘character_set_server’? And
> is ‘@‘ a legal part of a variable name?
>
> Also, is it handled by the core SQL parser or by a preprocessor?
>
> Julian
>
>
> > On Mar 22, 2022, at 2:17 AM, Adolfo Ochagavía <ado...@ochagavia.nl>
> wrote:
> >
> > Hi there,
> >
> > I am writing a MySQL-compatible server that talks the MySQL protocol.
> Some clients are sending special queries to autoconfigure themselves, like
> "SELECT @@character_set_server". I would like to use calcite to parse such
> queries, but parsing fails with an exception, seemingly related to the
> usage of "@@" in variable names. Is this unsupported or am I doing
> something wrong?
> >
> > The code:
> >> var config = SqlParser.Config.DEFAULT.withLex(Lex.MYSQL);
> >> var parser = SqlParser.create("SELECT @@character_set_server", config);
> >> var parsed = parser.parseQuery();
> >
> > The exception: org.apache.calcite.sql.parser.SqlParseException: Lexical
> error at line 1, column 9. Encountered: "@" (64), after : ""
> >
> > Any help is appreciated!
> > Adolfo
>
>
