Support session variable

[lonng]

No description provided.

[tanqiangyes]

pls add some description @lonng

[lonng]

You can reference MySQL session variable syntax. https://dev.mysql.com/doc/refman/5.6/en/set-variable.html

1. We need to add a new SET syntax
2. Add a session variable struct in the package stmtctx to store all session variables.

[tanqiangyes]

You can reference MySQL session variable syntax. https://dev.mysql.com/doc/refman/5.6/en/set-variable.html

1. We need to add a new SET syntax
2. Add a session variable struct in the package stmtctx to store all session variables.

ok, by the way, Please close the completed issue