MDEV-9077 add sys_schema from https://github.com/mysql/mysql-sys

scripts/sys_schema/functions/sys_get_config.sql

@@ -0,0 +1,98 @@
+-- Copyright (c) 2014, 2015, Oracle and/or its affiliates. All rights reserved.
+--
+-- This program is free software; you can redistribute it and/or modify
+-- it under the terms of the GNU General Public License as published by
+-- the Free Software Foundation; version 2 of the License.
+--
+-- This program is distributed in the hope that it will be useful,
+-- but WITHOUT ANY WARRANTY; without even the implied warranty of
+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+-- GNU General Public License for more details.
+--
+-- You should have received a copy of the GNU General Public License
+-- along with this program; if not, write to the Free Software
+-- Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+
+DROP FUNCTION IF EXISTS sys_get_config;
+
+DELIMITER $$
+
+CREATE DEFINER='root'@'localhost' FUNCTION sys_get_config (
+        in_variable_name VARCHAR(128),
+        in_default_value VARCHAR(128)
+    )
+    RETURNS VARCHAR(128)
+    COMMENT '
+             Description
+             -----------
+
+             Returns the value for the requested variable using the following logic:
+
+                1. If the option exists in sys.sys_config return the value from there.
+                2. Else fall back on the provided default value.
+
+             Notes for using sys_get_config():
+
+                * If the default value argument to sys_get_config() is NULL and case 2. is reached, NULL is returned.
+                  It is then expected that the caller is able to handle NULL for the given configuration option.
+                * The convention is to name the user variables @sys.<name of variable>. It is <name of variable> that
+                  is stored in the sys_config table and is what is expected as the argument to sys_get_config().
+                * If you want to check whether the configuration option has already been set and if not assign with
+                  the return value of sys_get_config() you can use IFNULL(...) (see example below). However this should
+                  not be done inside a loop (e.g. for each row in a result set) as for repeated calls where assignment
+                  is only needed in the first iteration using IFNULL(...) is expected to be significantly slower than
+                  using an IF (...) THEN ... END IF; block (see example below).
+
+             Parameters
+             -----------
+
+             in_variable_name (VARCHAR(128)):
+               The name of the config option to return the value for.
+
+             in_default_value (VARCHAR(128)):
+               The default value to return if the variable does not exist in sys.sys_config.
+
+             Returns
+             -----------
+
+             VARCHAR(128)
+
+             Example
+             -----------
+
+             -- Get the configuration value from sys.sys_config falling back on 128 if the option is not present in the table.
+             mysql> SELECT sys.sys_get_config(''statement_truncate_len'', 128) AS Value;
+             +-------+
+             | Value |
+             +-------+
+             | 64    |
+             +-------+
+             1 row in set (0.00 sec)
+
+             -- Check whether the option is already set, if not assign - IFNULL(...) one liner example.
+             mysql> SET @sys.statement_truncate_len = IFNULL(@sys.statement_truncate_len, sys.sys_get_config(''statement_truncate_len'', 64));
+             Query OK, 0 rows affected (0.00 sec)
+
+             -- Check whether the option is already set, if not assign - IF ... THEN ... END IF example.
+             IF (@sys.statement_truncate_len IS NULL) THEN
+                 SET @sys.statement_truncate_len = sys.sys_get_config(''statement_truncate_len'', 64);
+             END IF;
+            '
+    SQL SECURITY INVOKER
+    DETERMINISTIC
+    READS SQL DATA
+BEGIN
+    DECLARE v_value VARCHAR(128) DEFAULT NULL;
+
+    -- Check if we have the variable in the sys.sys_config table
+    SET v_value = (SELECT value FROM sys.sys_config WHERE variable = in_variable_name);
+  
+    -- Protection against the variable not existing in sys_config
+    IF (v_value IS NULL) THEN
+        SET v_value = in_default_value;
+    END IF;
+
+    RETURN v_value;
+END $$
+
+DELIMITER ;