Difference between revisions of "ORA-08177: can't serialize access for this transaction"

Latest revision as of 16:44, 6 February 2020

Reference

My Oracle Support Note : Insert Into Table Fails With ORA-08177 (Doc ID 160593.1)

Introduction

The ORA-8177 error "Cannot serialize access" is caused by the default block-level locking of Oracle's SERIALIZABLE isolation level. In the SERIALIZABLE isolation level, data "freezes" when a transaction starts. The data that the transaction is selecting cannot be modified by any other transaction; it is effectively locked for modification. Transactions then are "queued" one after another, and a transaction can proceed only when the transaction ahead of it has completed. Data readers are not blocked from selecting the data, however. In effect, every transaction locks every block it touches from further modification. Because the entire block is locked, all the rows in it are locked as well—even those that are not supposed to be.

Oracle 10g introduced a new feature called ROWDEPENDENCIES, which developers can leverage not only to avoid the dreaded ORA-8177 error but also to improve application concurrency. When this feature is used (for a specific table) data is no longer locked at the block-level, but at the row level, when Oracle's transaction isolation level is set to SERIALIZABLE.

Error Explanation

Symptoms:

•symptom: Insert into table fails

•symptom: ORA-08177: can't serialize access for this transaction

•symptom: transaction isolation level is serializable

•symptom: The table has index definitions

•symptom: Several transaction insert data into same table

•cause: The problem manifest itself when there are indexes on the table   
 to which the rows are being inserted.  Without any indexes the problem will not reproduce     

Cause:
Concurrent inserts into a table with index using serializable transactions could lead to an index split that the transaction tries to re-read and cause an error ORA-08177

Reproduce the error

First, as use robla, create the test objects:

SQL> create table emp2 as select * from emp where rownum < 1;

Table created.

SQL> create index emp2_indx on emp2(empno); 

Index created.

In two separate SQL sessions, as user robla, enter the following commands, i.e. type each individual command on one session followed immediately on the other session. (Do not connect as sys as you can't set the transaction isolation level as sys):

-- Session 1:
SQL_01> set transaction isolation level serializable;

Transaction set.

SQL> insert into emp2 select * from emp;

14 rows created.

-- Sesssion 2:
SQL_02> set transaction isolation level serializable;

SQL> insert into emp2 select * from emp;

14 rows created.

-- Session 1:
SQL_01> set transaction isolation level serializable;

Transaction set.

SQL> insert into emp2 select * from emp;

ERROR at line 1:
ORA-08177: can't serialize access for this transaction

Solutions

• Insert a smaller number of rows (e.g commit more often)
• or Catch the error code in the application and re-try the operation again
• this can be fixed by implementing "rowdependencies" during normal table creation.

Implement the 3rd solution

Explanation

This rowdependencies clause changes the default behavior of Oracle to put an SCN on every row within the tables, and not just one SCN for every physical data block. This adds a hidden column to each row, taking 6 bytes and storing the SCN on a row-by-row basis with your data. NOTE: This is a CREATE TABLE option only. You can’t modify a table to add rowdependencies, and there are a few restrictions for tables where this is enabled. Check the documentation for your version.

Implement the solution

First check the current table setting:

SQL> SELECT owner, table_name, dependencies FROM dba_tables where table_name = 'EMP2';                        

OWNER        TABLE_NAME DEPENDEN
------------ ---------- --------
ROBLA        EMP2       DISABLED

Drop and recreate the table, with ROWDEPENDENCIES: (you can't change it in an existing table unfortunately)

SQL> create table robla.emp2 (
EMPNO           NUMBER(4),
ENAME       VARCHAR2(10),
JOB         VARCHAR2(9),
MGR         NUMBER(4),
HIREDATE    DATE,
SAL         NUMBER(7,2),
COMM        NUMBER(7,2),
DEPTNO      NUMBER(2)
) ROWDEPENDENCIES
/  

Table created.

SQL> create index robla.emp2_indx on robla.emp2(empno); 

Index created.

Let's check the results:

SQL> SELECT owner, table_name, dependencies FROM dba_tables where table_name = 'EMP2'; 

OWNER        TABLE_NAME DEPENDEN
------------ ---------- --------
ROBLA        EMP2       ENABLED

Now re-execute the statements from before, and the error will no longer occur.

-- Session 1:
SQL> set transaction isolation level serializable;

Transaction set.

ROBLA @ TSTDB [T52-d1-db2-10g-sup] : 
SQL> insert into emp2 select * from emp;

14 rows created.

ROBLA @ TSTDB [T52-d1-db2-10g-sup] : 
SQL> insert into emp2 select * from emp;

14 rows created.

ROBLA @ TSTDB [T52-d1-db2-10g-sup] : 
SQL> /

14 rows created.

ROBLA @ TSTDB [T52-d1-db2-10g-sup] : 
SQL> /

14 rows created.

-- Session 2
SQL> set transaction isolation level serializable;

Transaction set.

ROBLA @ TSTDB [T52-d1-db2-10g-sup] : 
SQL> insert into emp2 select * from emp;

14 rows created.

ROBLA @ TSTDB [T52-d1-db2-10g-sup] : 
SQL> insert into emp2 select * from emp;

14 rows created.

ROBLA @ TSTDB [T52-d1-db2-10g-sup] : 
SQL> /

14 rows created.

ROBLA @ TSTDB [T52-d1-db2-10g-sup] : 
SQL> /

14 rows created.