FATAL：cache lookup failed for access method

[postgres@pg03 ~]$ psql -h 192.168.1.3 -U postgres -d tdb
psql: FATAL:  cache lookup failed for access method 403
使用客户端新建连接访问数据库时出现报错，无法建立连接，而访问其他数据库正常。
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postgresql后端服务进程在初始化阶段加载系统字典表时，由于系统字典表pg_am损坏导致加载失败，初始化失败报错退出。

PG后端服务进程在被fork出来之后会进行如下函数调用：
PostgresMain -> InitPostgres -> RelationCacheInitializePhase3 -> 
load_critical_index -> RelationBuildDesc -> RelationInitIndexAccessInfo
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其中，load_critical_index加载索引的顺序如下：
 load_critical_index(ClassOidIndexId,
                                                        RelationRelationId);
                load_critical_index(AttributeRelidNumIndexId,
                                                        AttributeRelationId);
                load_critical_index(IndexRelidIndexId,
                                                        IndexRelationId);
                load_critical_index(OpclassOidIndexId,
                                                        OperatorClassRelationId);
                load_critical_index(AccessMethodProcedureIndexId,
                                                        AccessMethodProcedureRelationId);
                load_critical_index(RewriteRelRulenameIndexId,
                                                        RewriteRelationId);
                load_critical_index(TriggerRelidNameIndexId,
                                                        TriggerRelationId);
 第一个加载的系统索引是ClassOidIndexId, OID为2662，查询系统表可看到如下信息：
 
 postgres=# select relname,relnamespace,relam from pg_class where oid=2662;         
      relname       | relnamespace | relam 
--------------------+--------------+-------
 pg_class_oid_index |           11 |   403
(1 row)
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postgres=# select oid,* from pg_am;
 oid  | amname |  amhandler  | amtype 
------+--------+-------------+--------
  403 | btree  | bthandler   | i
  405 | hash   | hashhandler | i
  783 | gist   | gisthandler | i
 2742 | gin    | ginhandler  | i
 4000 | spgist | spghandler  | i
 3580 | brin   | brinhandler | i
(6 rows)
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该索引所使用的正是报错中所提及的access method 403.btree索引。
最终报错出现在RelationInitIndexAccessInfo函数中如下：
...
 /*
         * Look up the index's access method, save the OID of its handler function
         */
        tuple = SearchSysCache1(AMOID, ObjectIdGetDatum(relation->rd_rel->relam));
        if (!HeapTupleIsValid(tuple))
                elog(ERROR, "cache lookup failed for access method %u",
                         relation->rd_rel->relam);
        aform = (Form_pg_am) GETSTRUCT(tuple);
        relation->rd_amhandler = aform->amhandler;
        ReleaseSysCache(tuple);
...
由于tdb库下pg_am（AMOID）系统字典表文件被损坏，导致初始化后端服务进程时无法将正确数据读取到内存结构中，因此查询不到相应值。
​
​
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如果没创建过额外的索引访问方法，如rum等第三方插件，则直接copy template0下的pg_am字典表文件到已损坏数据库目录下即可，否则需要额外创建数据库，将第三方插件安装之后再copy新建数据库下的pg_am字典表文件到已损坏数据库目录下。
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[postgres@pg03 base]$ psql -d tdb
psql: FATAL:  cache lookup failed for access method 403
[postgres@pg03 base]$ cp -r 13260/2601 399872/ 
[postgres@pg03 base]$ psql -d tdb             
psql (11.5 )
Type "help" for help.
​
tdb=#