Postgresql ERROR: database is not accepting commands to avoid wraparound data loss in database
ERROR: database is not accepting commands to avoid wraparound data loss in database "mydb"
HINT: Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions
可以看到是一些临时表的age比较大，应该是由于事物没有及时提交，影响到了数据库autovacuum这些表.
mydb=> select b.nspname,a.relname,a.relfrozenxid,age(a.relfrozenxid) from pg_class a, pg_namespace b where a.relnamespace=b.oid and a.relkind='r' order by a.relfrozenxid::text::int8 limit 10;
nspname | relname | relfrozenxid | age
-------------+----------------------------------+--------------+------------
pg_temp_51 | avcp_shower_online_log_tmp | 1185660983 | 1903192163
pg_temp_161 | tmp_avcp_mobile_phone_data | 2811680917 | 277172229
pg_temp_464 | avcp_shower_online_log_tmp | 2854092324 | 234760822
mydb=> select datname,age(datfrozenxid) from pg_database where datname=current_database();
datname | age
-----------+------------
avcp_work | 2146485849
这里由于我进入单用户模式进行过vacuum，所以最大age表的age不等于数据库的age了，vacuum之前是相等的.
通过以下语句可以查找出age年龄大于vacuum_freeze_table_age的表：
select datname,age(datfrozenxid) from pg_database where datname notin ('postgres','template0','template1') and age(datfrozenxid)>(selectsetting::int from pg_settings where name='vacuum_freeze_table_age')order by age(datfrozenxid) desc;
xidWarnLimit = xidStopLimit - 10000000 //如果xid上限达到xidWarnLimit，数据库会发出warning，让你进行vacuum
xidStopLimit = xidWrapLimit - 1000000 //如果xid上限达到xidStopLimit，数据库会不允许任何会话申请事物号，必须进入single mode，进行vacuum freeze
vi src/include/access/transam.h //MaxTransactionId在这里定义最大值为2^32
#define MaxTransactionId ((TransactionId) 0xFFFFFFFF)
xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1) //xid是一个32位值，所以2^32右移一位
xidVacLimit = oldest_datfrozenxid + autovacuum_freeze_max_age //如果xid达到autovacuum_freeze_max_age这个值，那么就算没开启自动vacuum，数据库也会进行vacuum
从以下代码可以看到报错原因：
src/backend/access/transam/varsup.c
if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
{
/*
* For safety's sake, we release XidGenLock while sending signals,
* warnings, etc. This is not so much because we care about
* preserving concurrency in this situation, as to avoid any
* possibility of deadlock while doing get_database_name(). First,
* copy all the shared values we'll need in this path.
*/
TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;
TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;
TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;
Oid oldest_datoid = ShmemVariableCache->oldestXidDB;
LWLockRelease(XidGenLock);
/*
* To avoid swamping the postmaster with signals, we issue the autovac
* request only once per 64K transaction starts. This still gives
* plenty of chances before we get into real trouble.
*/
if (IsUnderPostmaster && (xid % 65536) == 0)
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
if (IsUnderPostmaster &&
TransactionIdFollowsOrEquals(xid, xidStopLimit))
{
char *oldest_datname = get_database_name(oldest_datoid);
/* complain even if that DB has disappeared */
if (oldest_datname)
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",
oldest_datname),
errhint("Stop the postmaster and vacuum that database in single-user mode.\n"
"You might also need to commit or roll back old prepared transactions.")));
else
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("database is not accepting commands to avoid wraparound data loss in database with OID %u",
oldest_datoid),
errhint("Stop the postmaster and vacuum that database in single-user mode.\n"
"You might also need to commit or roll back old prepared transactions.")));
}
else if (TransactionIdFollowsOrEquals(xid, xidWarnLimit))
{
char *oldest_datname = get_database_name(oldest_datoid);
/* complain even if that DB has disappeared */
if (oldest_datname)
ereport(WARNING,
(errmsg("database \"%s\" must be vacuumed within %u transactions",
oldest_datname,
xidWrapLimit - xid),
errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
"You might also need to commit or roll back old prepared transactions.")));
else
ereport(WARNING,
(errmsg("database with OID %u must be vacuumed within %u transactions",
oldest_datoid,
xidWrapLimit - xid),
errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
"You might also need to commit or roll back old prepared transactions.")));
}
解决方法：
postgres --single -D $PGDATA mydb
如果mydb比较大，可能会很慢，可以找出最大年龄的表vaccum，如下：
vacuum freeze tablename;
如果数据库不大，可以直接vacuum freeze;做整个库的vacuum.