pg中关于AccessShareLock和ExclusiveLock的问题

HI，要搞清楚锁，首先要搞清楚，PG是有很多可以加锁的对象的，每种对象下面，再去看它的锁冲突。
可以加锁的对象：

        LOCKTAG_RELATION,                       /* whole relation */
        /* ID info for a relation is DB OID + REL OID; DB OID = 0 if shared */
        LOCKTAG_RELATION_EXTEND,        /* the right to extend a relation */
        /* same ID info as RELATION */
        LOCKTAG_PAGE,                           /* one page of a relation */
        /* ID info for a page is RELATION info + BlockNumber */
        LOCKTAG_TUPLE,                          /* one physical tuple */
        /* ID info for a tuple is PAGE info + OffsetNumber */
        LOCKTAG_TRANSACTION,            /* transaction (for waiting for xact done) */
        /* ID info for a transaction is its TransactionId */
        LOCKTAG_VIRTUALTRANSACTION, /* virtual transaction (ditto) */
        /* ID info for a virtual transaction is its VirtualTransactionId */
        LOCKTAG_SPECULATIVE_TOKEN,      /* speculative insertion Xid and token */
        /* ID info for a transaction is its TransactionId */
        LOCKTAG_OBJECT,                         /* non-relation database object */
        /* ID info for an object is DB OID + CLASS OID + OBJECT OID + SUBID */

        /*
         * Note: object ID has same representation as in pg_depend and
         * pg_description, but notice that we are constraining SUBID to 16 bits.
         * Also, we use DB OID = 0 for shared objects such as tablespaces.
         */
        LOCKTAG_USERLOCK,                       /* reserved for old contrib/userlock code */
        LOCKTAG_ADVISORY                        /* advisory user locks */

锁模式

/* NoLock is not a lock mode, but a flag value meaning "don't get a lock" */
#define NoLock                                  0

#define AccessShareLock                 1               /* SELECT */
#define RowShareLock                    2               /* SELECT FOR UPDATE/FOR SHARE */
#define RowExclusiveLock                3               /* INSERT, UPDATE, DELETE */
#define ShareUpdateExclusiveLock 4              /* VACUUM (non-FULL),ANALYZE, CREATE
                                                                                 * INDEX CONCURRENTLY */
#define ShareLock                               5               /* CREATE INDEX (WITHOUT CONCURRENTLY) */
#define ShareRowExclusiveLock   6               /* like EXCLUSIVE MODE, but allows ROW
                                                                                 * SHARE */
#define ExclusiveLock                   7               /* blocks ROW SHARE/SELECT...FOR
                                                                                 * UPDATE */
#define AccessExclusiveLock             8               /* ALTER TABLE, DROP TABLE, VACUUM
                                                                                 * FULL, and unqualified LOCK TABLE */

然后我们说一下死锁，死锁相互等待造成的，你给的图没有等待，所以一定不是死锁。
最后提供一个查询锁和等待的方法给你，会比较好看。

用一个函数来将锁转换为数字，
postgres=# create or replace function f_lock_level(i_mode text) returns int as 
$$

declare
begin
  case i_mode
    when 'INVALID' then return 0;
    when 'AccessShareLock' then return 1;
    when 'RowShareLock' then return 2;
    when 'RowExclusiveLock' then return 3;
    when 'ShareUpdateExclusiveLock' then return 4;
    when 'ShareLock' then return 5;
    when 'ShareRowExclusiveLock' then return 6;
    when 'ExclusiveLock' then return 7;
    when 'AccessExclusiveLock' then return 8;
    else return 0;
  end case;
end; 

$$
 language plpgsql strict;
修改查询语句，按锁级别排序：
with t_wait as                     
(select a.mode,a.locktype,a.database,a.relation,a.page,a.tuple,a.classid,a.objid,a.objsubid,
a.pid,a.virtualtransaction,a.virtualxid,a,transactionid,b.query,b.xact_start,b.query_start,
b.usename,b.datname from pg_locks a,pg_stat_activity b where a.pid=b.pid and not a.granted),
t_run as 
(select a.mode,a.locktype,a.database,a.relation,a.page,a.tuple,a.classid,a.objid,a.objsubid,
a.pid,a.virtualtransaction,a.virtualxid,a,transactionid,b.query,b.xact_start,b.query_start,
b.usename,b.datname from pg_locks a,pg_stat_activity b where a.pid=b.pid and a.granted) 
select r.locktype,r.mode r_mode,r.usename r_user,r.datname r_db,r.relation::regclass,r.pid r_pid,
r.page r_page,r.tuple r_tuple,r.xact_start r_xact_start,r.query_start r_query_start,
now()-r.query_start r_locktime,r.query r_query,w.mode w_mode,w.pid w_pid,w.page w_page,
w.tuple w_tuple,w.xact_start w_xact_start,w.query_start w_query_start,
now()-w.query_start w_locktime,w.query w_query  
from t_wait w,t_run r where
  r.locktype is not distinct from w.locktype and
  r.database is not distinct from w.database and
  r.relation is not distinct from w.relation and
  r.page is not distinct from w.page and
  r.tuple is not distinct from w.tuple and
  r.classid is not distinct from w.classid and
  r.objid is not distinct from w.objid and
  r.objsubid is not distinct from w.objsubid and
  r.transactionid is not distinct from w.transactionid and
  r.pid <> w.pid
  order by f_lock_level(w.mode)+f_lock_level(r.mode) desc,r.xact_start;
现在可以排在前面的就是锁级别高的等待，优先干掉这个。
-[ RECORD 1 ]-+---------------------------------------------------------------------
locktype      | relation  -- 冲突类型
r_mode        | ShareUpdateExclusiveLock  -- 持锁模式
r_user        | postgres  -- 持锁用户
r_db          | postgres  -- 持锁数据库
relation      | tbl  -- 持锁对象
r_pid         | 25656  -- 持锁进程
r_xact_start  | 2015-05-10 14:11:16.08318+08  -- 持锁事务开始时间
r_query_start | 2015-05-10 14:11:16.08318+08  -- 持锁SQL开始时间
r_locktime    | 00:01:49.460779  -- 持锁时长
r_query       | vacuum freeze tbl;  --  持锁SQL,注意不一定是这个SQL带来的锁,也有可能是这个事务在之前执行的SQL加的锁
w_mode        | AccessExclusiveLock  -- 等待锁模式
w_pid         | 26731  -- 等待锁进程
w_xact_start  | 2015-05-10 14:11:17.987362+08  --  等待锁事务开始时间
w_query_start | 2015-05-10 14:11:17.987362+08  --  等待锁SQL开始时间
w_locktime    | 00:01:47.556597  --  等待锁时长
w_query       | truncate tbl;  -- 等待锁SQL
-[ RECORD 2 ]-+---------------------------------------------------------------------
locktype      | relation
r_mode        | ShareUpdateExclusiveLock
r_user        | postgres
r_db          | postgres
relation      | tbl
r_pid         | 25656
r_xact_start  | 2015-05-10 14:11:16.08318+08
r_query_start | 2015-05-10 14:11:16.08318+08
r_locktime    | 00:01:49.460779
r_query       | vacuum freeze tbl;
w_mode        | RowExclusiveLock
w_pid         | 25582
w_xact_start  | 2015-05-10 14:11:22.845+08
w_query_start | 2015-05-10 14:11:22.845+08
w_locktime    | 00:01:42.698959
w_query       | insert into tbl(crt_time) select now() from generate_series(1,1000);  -- 这个SQL其实等待的是truncate tbl的锁;
......