Oracle Copy Execution Plan from One Database to Other - oracle

Question

I have two identical databases.
On Database 1, a query gives response almost instantly.
On Database 2 - the execution plan is much different and query takes forever.
How can I copy the execution plan from Database 1 and force the optimizer on Database 2 to use that query plan ?

Answer 1

The fastest way to copy an execution plan is to use a hint outline.
First, find or generate the execution plan:
explain plan for select * from dual;
Second, display the execution plan with the +outline format option:
select * from table(dbms_xplan.display(format => '+outline'));
Plan hash value: 272002086
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 2 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| DUAL | 1 | 2 | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------
Outline Data
-------------
/*+
BEGIN_OUTLINE_DATA
FULL(#"SEL$1" "DUAL"#"SEL$1")
OUTLINE_LEAF(#"SEL$1")
ALL_ROWS
DB_VERSION('12.2.0.1')
OPTIMIZER_FEATURES_ENABLE('12.2.0.1')
IGNORE_OPTIM_EMBEDDED_HINTS
END_OUTLINE_DATA
*/
Finally, use the entire hint in the top-level of the SQL statement, like this:
select
/*+
BEGIN_OUTLINE_DATA
FULL(#"SEL$1" "DUAL"#"SEL$1")
OUTLINE_LEAF(#"SEL$1")
ALL_ROWS
DB_VERSION('12.2.0.1')
OPTIMIZER_FEATURES_ENABLE('12.2.0.1')
IGNORE_OPTIM_EMBEDDED_HINTS
END_OUTLINE_DATA
*/
*
from dual;
Hints are directives that are obeyed if possible. If one of the dependent objects, like an index, is unavailable then the hints may not be obeyed.
Forcing execution plans like this can be a fast way to fix a performance problem, but it doesn't fix the root cause. It's normally better to try to figure out why two databases are generating different plans. But that is a much more difficult problem to solve.