SQL: allow FOR SYSTEM_TIME BEFORE for SELECT queries [closes #170]

mysql-test/suite/versioning/r/truncate_history.result

@@ -122,6 +122,9 @@ a
 22
 1
 2
+select * from t for system_time before timestamp @ts1;
+a
+1
 truncate t for system_time before timestamp @ts1;
 select * from t for system_time all;
 a
@@ -144,6 +147,12 @@ a
 22
 1
 2
+select * from t for system_time before timestamp @ts1;
+a
+select * from t for system_time before transaction 2000;
+a
+1
+2
 truncate t for system_time before timestamp @ts1;
 select * from t for system_time all;
 a
@@ -157,7 +166,5 @@ a
 11
 22
 2
-select * from t for system_time before timestamp now();
-ERROR HY000: `FOR SYSTEM_TIME BEFORE` works only with `TRUNCATE` query type
 drop table t;
 drop procedure truncate_history_of_t;

mysql-test/suite/versioning/t/truncate_history.test

@@ -96,6 +96,7 @@ update t set a=11 where a=1;
 set @ts1=now(6);
 update t set a=22 where a=2;
 select * from t for system_time all;
+select * from t for system_time before timestamp @ts1;
 truncate t for system_time before timestamp @ts1;
 select * from t for system_time all;
 truncate t for system_time before timestamp now(6);
@@ -107,13 +108,12 @@ update t set a=11 where a=1;
 set @ts1=now(6);
 update t set a=22 where a=2;
 select * from t for system_time all;
+select * from t for system_time before timestamp @ts1;
+select * from t for system_time before transaction 2000;
 truncate t for system_time before timestamp @ts1;
 select * from t for system_time all;
 truncate t for system_time before timestamp now(6);
 select * from t for system_time all;
 
---error ER_VERS_WRONG_QUERY_TYPE
-select * from t for system_time before timestamp now();
-
 drop table t;
 drop procedure truncate_history_of_t;

sql/sql_select.cc

@@ -786,14 +786,6 @@ int vers_setup_select(THD *thd, TABLE_LIST *tables, COND **where_expr,
         table->vers_conditions.type == FOR_SYSTEM_TIME_UNSPECIFIED ?
           slex->vers_conditions : table->vers_conditions;
 
-      if (vers_conditions.type == FOR_SYSTEM_TIME_BEFORE &&
-          thd->lex->sql_command != SQLCOM_TRUNCATE)
-      {
-        my_error(ER_VERS_WRONG_QUERY_TYPE, MYF(0), "FOR SYSTEM_TIME BEFORE",
-                 "TRUNCATE");
-        DBUG_RETURN(-1);
-      }
-
       if (vers_conditions.type == FOR_SYSTEM_TIME_UNSPECIFIED)
       {
         if (vers_conditions.init_from_sysvar(thd))