PostgreSQL 源码解读（118）- MVCC#3(Tuple可见性判断)

//事务ID,无符号32位整型 typedef uint32 TransactionId; /* ---------------- * Special transaction ID values * 系统保留的事务ID * * BootstrapTransactionId is the XID for "bootstrap" operations, and * FrozenTransactionId is used for very old tuples. Both should * always be considered valid. * * FirstNormalTransactionId is the first "normal" transaction id. * Note: if you need to change it, you must change pg_class.h as well. * ---------------- */ #define InvalidTransactionId ((TransactionId) 0) #define BootstrapTransactionId ((TransactionId) 1) #define FrozenTransactionId ((TransactionId) 2) #define FirstNormalTransactionId ((TransactionId) 3) #define MaxTransactionId ((TransactionId) 0xFFFFFFFF) /* ---------------- * transaction ID manipulation macros * ---------------- */ #define TransactionIdIsValid(xid) ((xid) != InvalidTransactionId) #define TransactionIdIsNormal(xid) ((xid) >= FirstNormalTransactionId) #define TransactionIdEquals(id1, id2) ((id1) == (id2)) #define TransactionIdStore(xid, dest) (*(dest) = (xid)) #define StoreInvalidTransactionId(dest) (*(dest) = InvalidTransactionId) /* advance a transaction ID variable, handling wraparound correctly */ #define TransactionIdAdvance(dest) \ do { \ (dest)++; \ if ((dest) < FirstNormalTransactionId) \ (dest) = FirstNormalTransactionId; \ } while(0) /* back up a transaction ID variable, handling wraparound correctly */ #define TransactionIdRetreat(dest) \ do { \ (dest)--; \ } while ((dest) < FirstNormalTransactionId) /* compare two XIDs already known to be normal; this is a macro for speed */ //比较两个已知是常规事务的XIDs;宏定义是为了性能考虑. #define NormalTransactionIdPrecedes(id1, id2) \ (AssertMacro(TransactionIdIsNormal(id1) && TransactionIdIsNormal(id2)), \ (int32) ((id1) - (id2)) < 0) /* compare two XIDs already known to be normal; this is a macro for speed */ #define NormalTransactionIdFollows(id1, id2) \ (AssertMacro(TransactionIdIsNormal(id1) && TransactionIdIsNormal(id2)), \ (int32) ((id1) - (id2)) > 0) /* * HeapTupleHeaderGetRawXmin returns the "raw" xmin field, which is the xid * originally used to insert the tuple. However, the tuple might actually * be frozen (via HeapTupleHeaderSetXminFrozen) in which case the tuple's xmin * is visible to every snapshot. Prior to PostgreSQL 9.4, we actually changed * the xmin to FrozenTransactionId, and that value may still be encountered * on disk. * HeapTupleHeaderGetRawXmin返回“原始”xmin字段，这是最初用于插入元组的xid。 * 但是，实际上元组实可能被冻结(通过HeapTupleHeaderSetXminFrozen)， * 在这种情况下，元组可对每个快照都可见。 * 在PostgreSQL 9.4之前，我们实际上将xmin更改为FrozenTransactionId，该值仍然可能在磁盘上遇到。 */ #define HeapTupleHeaderGetRawXmin(tup) \ ( \ (tup)->t_choice.t_heap.t_xmin \ ) #define HeapTupleHeaderXminCommitted(tup) \ ( \ ((tup)->t_infomask & HEAP_XMIN_COMMITTED) != 0 \ ) /* * HeapTupleHeaderGetRawXmax gets you the raw Xmax field. To find out the Xid * that updated a tuple, you might need to resolve the MultiXactId if certain * bits are set. HeapTupleHeaderGetUpdateXid checks those bits and takes care * to resolve the MultiXactId if necessary. This might involve multixact I/O, * so it should only be used if absolutely necessary. * HeapTupleHeaderGetRawXmax返回原始的Xmax字段. * 搜索更新元组的事务Xid,如设定了特定的标记位则可能需要解决MultiXactId. * HeapTupleHeaderGetUpdateXid检查这些标记位并在需要时解决MultiXactId问题. * 这可能会引起multixact I/O,如非必要不要使用. */ #define HeapTupleHeaderGetUpdateXid(tup) \ ( \ (!((tup)->t_infomask & HEAP_XMAX_INVALID) && \ ((tup)->t_infomask & HEAP_XMAX_IS_MULTI) && \ !((tup)->t_infomask & HEAP_XMAX_LOCK_ONLY)) ? \ HeapTupleGetUpdateXid(tup) \ : \ HeapTupleHeaderGetRawXmax(tup) \ ) #define HeapTupleHeaderGetRawXmax(tup) \ ( \ (tup)->t_choice.t_heap.t_xmax \ ) /* * A tuple is only locked (i.e. not updated by its Xmax) if the * HEAP_XMAX_LOCK_ONLY bit is set; or, for pg_upgrade's sake, if the Xmax is * not a multi and the EXCL_LOCK bit is set. * 如HEAP_XMAX_LOCK_ONLY标记位设置,则表示仅仅只是锁定了tuple(比如并非更新了Xmax字段). * 或者,对于pg_upgrade升级,如Xmax并非HEAP_XMAX_IS_MULTI而且EXCL_LOCK标记位设置,也是表达这个意思. * * See also HeapTupleHeaderIsOnlyLocked, which also checks for a possible * aborted updater transaction. * 可同时参考HeapTupleHeaderIsOnlyLocked,该函数也检查了可能已回滚的更新事务. * * Beware of multiple evaluations of the argument. * 注意对参数的多种分析. */ #define HEAP_XMAX_IS_LOCKED_ONLY(infomask) \ (((infomask) & HEAP_XMAX_LOCK_ONLY) || \ (((infomask) & (HEAP_XMAX_IS_MULTI | HEAP_LOCK_MASK)) == HEAP_XMAX_EXCL_LOCK))

/* * HeapTupleSatisfiesMVCC * True iff heap tuple is valid for the given MVCC snapshot. * HeapTupleSatisfiesMVCC -- 如Tuple对于给定的MVCC快照可见,则返回T * * Here, we consider the effects of: * all transactions committed as of the time of the given snapshot * previous commands of this transaction * 在这里，我们考虑的是: * 在给定快照之前的事务命令执行时提交的所有事务 * * Does _not_ include: * transactions shown as in-progress by the snapshot * transactions started after the snapshot was taken * changes made by the current command * 但不包括: * 快照显示仍在进行中的事务. * 快照之后才启动的事务. * * * Notice that here, we will not update the tuple status hint bits if the * inserting/deleting transaction is still running according to our snapshot, * even if in reality it's committed or aborted by now. This is intentional. * Checking the true transaction state would require access to high-traffic * shared data structures, creating contention we'd rather do without, and it * would not change the result of our visibility check anyway. The hint bits * will be updated by the first visitor that has a snapshot new enough to see * the inserting/deleting transaction as done. In the meantime, the cost of * leaving the hint bits unset is basically that each HeapTupleSatisfiesMVCC * call will need to run TransactionIdIsCurrentTransactionId in addition to * XidInMVCCSnapshot (but it would have to do the latter anyway). In the old * coding where we tried to set the hint bits as soon as possible, we instead * did TransactionIdIsInProgress in each call --- to no avail, as long as the * inserting/deleting transaction was still running --- which was more cycles * and more contention on the PGXACT array. * 在这里需要注意的是,如果插入/删除事务仍然在快照范围内运行， * 那么我们将不会更新元组状态提示位，即使它现在已经提交或中止。这是做法内部约定的. * 检查真正的事务状态需要访问高频访问的共享数据结构， * 这就产生了我们不希望看到的争用，而且它也不会改变可见性检查的结果。 * 第一个访问者将更新提示位，该访问者的快照足够新，可以看到插入/删除事务已经完成。 * 与此同时，不设置提示位的代价是，除了XidInMVCCSnapshot之外， * 每个HeapTupleSatisfiesMVCC调用都需要运行TransactionIdIsCurrentTransactionId * (无论如何都必须执行后者)。 * 在旧的编码中，我们试图尽快地设置提示位， * 而不是在每次调用中执行TransactionIdIsInProgress—— * 只要插入/删除事务仍在运行,这些工作看起来都是徒劳的 * ——导致PGXACT数组上更多的循环和争用。 */ bool HeapTupleSatisfiesMVCC(HeapTuple htup, Snapshot snapshot, Buffer buffer) { HeapTupleHeader tuple = htup->t_data;//Tuple Assert(ItemPointerIsValid(&htup->t_self)); Assert(htup->t_tableOid != InvalidOid); if (!HeapTupleHeaderXminCommitted(tuple)) { //A.xmin事务未提交(HEAP_XMIN_COMMITTED标记未设置) if (HeapTupleHeaderXminInvalid(tuple)) //xmin = 0,此tuple不可见 return false; /* Used by pre-9.0 binary upgrades */ //用于9.0前二进制可执行文件的升级,HEAP_MOVED_OFF & HEAP_MOVED_IN已不再使用 if (tuple->t_infomask & HEAP_MOVED_OFF) { TransactionId xvac = HeapTupleHeaderGetXvac(tuple); if (TransactionIdIsCurrentTransactionId(xvac)) return false; if (!XidInMVCCSnapshot(xvac, snapshot)) { if (TransactionIdDidCommit(xvac)) { SetHintBits(tuple, buffer, HEAP_XMIN_INVALID, InvalidTransactionId); return false; } SetHintBits(tuple, buffer, HEAP_XMIN_COMMITTED, InvalidTransactionId); } } /* Used by pre-9.0 binary upgrades */ //同上 else if (tuple->t_infomask & HEAP_MOVED_IN) { TransactionId xvac = HeapTupleHeaderGetXvac(tuple); if (!TransactionIdIsCurrentTransactionId(xvac)) { if (XidInMVCCSnapshot(xvac, snapshot)) return false; if (TransactionIdDidCommit(xvac)) SetHintBits(tuple, buffer, HEAP_XMIN_COMMITTED, InvalidTransactionId); else { SetHintBits(tuple, buffer, HEAP_XMIN_INVALID, InvalidTransactionId); return false; } } } else if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tuple))) { //A.xmin事务未提交 //A1.xmin对应的事务是当前事务 if (HeapTupleHeaderGetCmin(tuple) >= snapshot->curcid) //A1-1.tuple.cmin >= 快照的cid,说明插入在扫描开始前,返回F return false; /* inserted after scan started */ if (tuple->t_infomask & HEAP_XMAX_INVALID) /* xid invalid */ //A1-2.xmax无效,返回T return true; if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask)) /* not deleter */ //A1-3.XMAX仅仅只是锁定而已,返回T return true; if (tuple->t_infomask & HEAP_XMAX_IS_MULTI) { //xmax is a MultiXactId TransactionId xmax; //获取更新的Xid xmax = HeapTupleGetUpdateXid(tuple); /* not LOCKED_ONLY, so it has to have an xmax */ //验证xmax是有效的 Assert(TransactionIdIsValid(xmax)); /* updating subtransaction must have aborted */ //执行更新的子事务必须已回滚(why?因为前提是xmin事务是未提交的,因此xmax必须已回滚) if (!TransactionIdIsCurrentTransactionId(xmax)) //正在执行删除的事务不是本事务,返回T return true; else if (HeapTupleHeaderGetCmax(tuple) >= snapshot->curcid) //本事务执行删除:删除命令在快照之后,返回T return true; /* updated after scan started */ else //本事务执行删除:删除命令在快照之前,返回F return false; /* updated before scan started */ } if (!TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmax(tuple))) { //A1-4.tuple.xmax对应的事务不是本事务 /* deleting subtransaction must have aborted */ //删除子事务已终止(why?因为前提是xmin事务是未提交的,因此xmax必须已回滚) SetHintBits(tuple, buffer, HEAP_XMAX_INVALID, InvalidTransactionId); return true; } if (HeapTupleHeaderGetCmax(tuple) >= snapshot->curcid) //A1-5.删除在快照之后,返回T return true; /* deleted after scan started */ else //A1-6.删在快照之前,返回F return false; /* deleted before scan started */ } else if (XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot)) //A.xmin事务未提交; //A2.xmin在当前快照中(非本事务) --> 不可见 return false; else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmin(tuple))) //A.xmin事务未提交; //A3.查询事务日志clog,Xmin事务已提交,则设置标志位,可见判断逻辑在后续执行 SetHintBits(tuple, buffer, HEAP_XMIN_COMMITTED, HeapTupleHeaderGetRawXmin(tuple)); else { /* it must have aborted or crashed */ //A4.以上各种情况都不符合,事务回滚或者在执行事务过程中crash了 //设置标志位,返回F SetHintBits(tuple, buffer, HEAP_XMIN_INVALID, InvalidTransactionId); return false; } } else { /* xmin is committed, but maybe not according to our snapshot */ //B.xmin事务已提交,但可能不属于该快照 if (!HeapTupleHeaderXminFrozen(tuple) && XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot)) return false; /* treat as still in progress */ } /* by here, the inserting transaction has committed */ //C.Xmin事务已提交 if (tuple->t_infomask & HEAP_XMAX_INVALID) /* xid invalid or aborted */ //C1.xmax无效(HEAP_XMAX_INVALID),则返回T return true; if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask)) //C2.xmax仅仅只是lock而已,返回T return true; if (tuple->t_infomask & HEAP_XMAX_IS_MULTI) { //xmax is a MultiXactId TransactionId xmax; /* already checked above */ //再次检查 Assert(!HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask)); //获取xmax xmax = HeapTupleGetUpdateXid(tuple); /* not LOCKED_ONLY, so it has to have an xmax */ Assert(TransactionIdIsValid(xmax)); if (TransactionIdIsCurrentTransactionId(xmax)) { //xmax为当前事务id if (HeapTupleHeaderGetCmax(tuple) >= snapshot->curcid) //快照之后执行删除,返回T return true; /* deleted after scan started */ else //快照之前执行删除,返回F return false; /* deleted before scan started */ } if (XidInMVCCSnapshot(xmax, snapshot)) //xmax仍在进行中,返回T return true; if (TransactionIdDidCommit(xmax)) //xmax事务通过查询clog日志,已确认提交,则返回F return false; /* updating transaction committed */ /* it must have aborted or crashed */ //xmax回滚或者执行期间crash,返回T return true; } if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED)) { //C3.xmax未提交 if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmax(tuple))) { //C3-1:本事务 if (HeapTupleHeaderGetCmax(tuple) >= snapshot->curcid) return true; /* deleted after scan started */ else return false; /* deleted before scan started */ } if (XidInMVCCSnapshot(HeapTupleHeaderGetRawXmax(tuple), snapshot)) //C3-2:非本事务 //xmax事务仍在进行中 return true; if (!TransactionIdDidCommit(HeapTupleHeaderGetRawXmax(tuple))) { /* it must have aborted or crashed */ //C3-3查询clog日志,xmax事务未提交(回滚或crash),设置标记 SetHintBits(tuple, buffer, HEAP_XMAX_INVALID, InvalidTransactionId); //可见 return true; } /* xmax transaction committed */ //以上判断均不成立,则可认为事务已提交,设置标记 SetHintBits(tuple, buffer, HEAP_XMAX_COMMITTED, HeapTupleHeaderGetRawXmax(tuple)); } else { /* xmax is committed, but maybe not according to our snapshot */ //C4.xmax已提交,但快照指示该事务仍在进行中 if (XidInMVCCSnapshot(HeapTupleHeaderGetRawXmax(tuple), snapshot)) //仍在处理中,返回T return true; /* treat as still in progress */ } /* xmax transaction committed */ //C5.xmax已提交,返回F return false; }

11:55:00 (xdb@[local]:5432)testdb=# drop table t_mvcc2; ERROR: table "t_mvcc2" does not exist 11:55:01 (xdb@[local]:5432)testdb=# create table t_mvcc2(c1 int not null,c2 varchar(40),c3 varchar(40)); CREATE TABLE 11:55:02 (xdb@[local]:5432)testdb=# 11:55:02 (xdb@[local]:5432)testdb=# insert into t_mvcc2(c1,c2,c3) values(1,'C2-1','C3-1'); INSERT 0 1 11:55:02 (xdb@[local]:5432)testdb=# insert into t_mvcc2(c1,c2,c3) values(2,'C2-2','C3-2'); INSERT 0 1 11:55:03 (xdb@[local]:5432)testdb=# 11:55:38 (xdb@[local]:5432)testdb=# begin; BEGIN 11:55:38 (xdb@[local]:5432)testdb=#* 11:55:38 (xdb@[local]:5432)testdb=#* delete from t_mvcc2 where c1 = 1; DELETE 1 11:55:38 (xdb@[local]:5432)testdb=#* 11:55:38 (xdb@[local]:5432)testdb=#* commit; COMMIT 11:55:38 (xdb@[local]:5432)testdb=# 11:55:38 (xdb@[local]:5432)testdb=# begin; BEGIN 11:55:38 (xdb@[local]:5432)testdb=#* 11:55:38 (xdb@[local]:5432)testdb=#* update t_mvcc2 set c2 = 'C2#'||substr(c2,4,40) where c1 = 2; UPDATE 1 11:55:38 (xdb@[local]:5432)testdb=#* 12:19:17 (xdb@[local]:5432)testdb=#* select txid_current(); txid_current -------------- 2363 (1 row)

11:58:45 (xdb@[local]:5432)testdb=# select lp,lp_off,lp_flags,t_xmin,t_xmax,t_field3 as t_cid,t_ctid,t_infomask2,t_infomask from heap_page_items(get_raw_page('t_mvcc2',0)); lp | lp_off | lp_flags | t_xmin | t_xmax | t_cid | t_ctid | t_infomask2 | t_infomask ----+--------+----------+--------+--------+-------+--------+-------------+------------ 1 | 8152 | 1 | 2360 | 2362 | 0 | (0,1) | 8195 | 1282 2 | 8112 | 1 | 2361 | 2363 | 0 | (0,3) | 16387 | 258 3 | 8072 | 1 | 2363 | 0 | 0 | (0,3) | 32771 | 10242 (3 rows) 11:59:38 (xdb@[local]:5432)testdb=# select * from t_mvcc2;

(gdb) b HeapTupleSatisfiesMVCC Breakpoint 1 at 0xa93125: file tqual.c, line 966. (gdb) c Continuing. Breakpoint 1, HeapTupleSatisfiesMVCC (htup=0x1985d18, snapshot=0x1a06358, buffer=69) at tqual.c:966 966 HeapTupleHeader tuple = htup->t_data; (gdb)

(gdb) bt #0 HeapTupleSatisfiesMVCC (htup=0x1985d18, snapshot=0x1a06358, buffer=69) at tqual.c:966 #1 0x00000000004de959 in heap_hot_search_buffer (tid=0x1985d1c, relation=0x7f002f73e2b0, buffer=69, snapshot=0x1a06358, heapTuple=0x1985d18, all_dead=0x7ffc56ed8862, first_call=true) at heapam.c:2127 #2 0x00000000004fd12e in index_fetch_heap (scan=0x1985cb8) at indexam.c:608 #3 0x00000000004fd35d in index_getnext (scan=0x1985cb8, direction=ForwardScanDirection) at indexam.c:691 #4 0x00000000004fb910 in systable_getnext (sysscan=0x19868f8) at genam.c:425 #5 0x0000000000a1cb63 in SearchCatCacheMiss (cache=0x19de480, nkeys=1, hashValue=1281076841, hashIndex=105, v1=42610, v2=0, v3=0, v4=0) at catcache.c:1386 #6 0x0000000000a1ca10 in SearchCatCacheInternal (cache=0x19de480, nkeys=1, v1=42610, v2=0, v3=0, v4=0) at catcache.c:1317 #7 0x0000000000a1c6fe in SearchCatCache1 (cache=0x19de480, v1=42610) at catcache.c:1185 #8 0x0000000000a37543 in SearchSysCache1 (cacheId=50, key1=42610) at syscache.c:1119 #9 0x0000000000a6ae60 in check_enable_rls (relid=42610, checkAsUser=0, noError=false) at rls.c:66 #10 0x000000000087b286 in get_row_security_policies (root=0x1985dd0, rte=0x1985ee8, rt_index=1, securityQuals=0x7ffc56ed8cc0, withCheckOptions=0x7ffc56ed8cb8, hasRowSecurity=0x7ffc56ed8cb7, hasSubLinks=0x7ffc56ed8cb6) at rowsecurity.c:133 #11 0x0000000000875e1c in fireRIRrules (parsetree=0x1985dd0, activeRIRs=0x0) at rewriteHandler.c:1904 #12 0x0000000000878b23 in QueryRewrite (parsetree=0x1985dd0) at rewriteHandler.c:3712 #13 0x00000000008c64d7 in pg_rewrite_query (query=0x1985dd0) at postgres.c:782 #14 0x00000000008c634e in pg_analyze_and_rewrite (parsetree=0x1985c20, query_string=0x1984ec8 "select * from t_mvcc2;", paramTypes=0x0, numParams=0, queryEnv=0x0) at postgres.c:698 #15 0x00000000008c6993 in exec_simple_query (query_string=0x1984ec8 "select * from t_mvcc2;") at postgres.c:1070 #16 0x00000000008cae70 in PostgresMain (argc=1, argv=0x19b0dc8, dbname=0x19b0c30 "testdb", username=0x1981ba8 "xdb") at postgres.c:4182 #17 0x000000000082642b in BackendRun (port=0x19a6c00) at postmaster.c:4361 #18 0x0000000000825b8f in BackendStartup (port=0x19a6c00) at postmaster.c:4033 #19 0x0000000000821f1c in ServerLoop () at postmaster.c:1706 #20 0x00000000008217b4 in PostmasterMain (argc=1, argv=0x197fb60) at postmaster.c:1379 ---Type <return> to continue, or q <return> to quit--- #21 0x00000000007488ef in main (argc=1, argv=0x197fb60) at main.c:228 (gdb)

(gdb) p *snapshot --> 快照信息 $3 = {satisfies = 0xa9310d <HeapTupleSatisfiesMVCC>, xmin = 2363, xmax = 2363, xip = 0x0, xcnt = 0, subxip = 0x0, subxcnt = 0, suboverflowed = false, takenDuringRecovery = false, copied = true, curcid = 0, speculativeToken = 0, active_count = 0, regd_count = 1, ph_node = {first_child = 0x0, next_sibling = 0x0, prev_or_parent = 0xf9bfa0 <CatalogSnapshotData+64>}, whenTaken = 0, lsn = 0}

(gdb) n 968 Assert(ItemPointerIsValid(&htup->t_self)); (gdb) p tuple $1 = (HeapTupleHeader) 0x7f0002da7a60 (gdb) p *tuple $2 = {t_choice = {t_heap = {t_xmin = 2359, t_xmax = 0, t_field3 = {t_cid = 0, t_xvac = 0}}, t_datum = {datum_len_ = 2359, datum_typmod = 0, datum_typeid = 0}}, t_ctid = {ip_blkid = {bi_hi = 0, bi_lo = 12}, ip_posid = 6}, t_infomask2 = 33, t_infomask = 2313, t_hoff = 32 ' ', t_bits = 0x7f0002da7a77 "\377\377\377?"} (gdb)

(gdb) n 1074 XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot)) (gdb) 1073 if (!HeapTupleHeaderXminFrozen(tuple) && (gdb) 1080 if (tuple->t_infomask & HEAP_XMAX_INVALID) /* xid invalid or aborted */ (gdb)

(gdb) p *htup $4 = {t_len = 172, t_self = {ip_blkid = {bi_hi = 0, bi_lo = 1}, ip_posid = 40}, t_tableOid = 1247, t_data = 0x7f0002e52800} (gdb) n 968 Assert(ItemPointerIsValid(&htup->t_self)); (gdb) p tuple $5 = (HeapTupleHeader) 0x7f0002e52800 (gdb) p *tuple $6 = {t_choice = {t_heap = {t_xmin = 1, t_xmax = 0, t_field3 = {t_cid = 0, t_xvac = 0}}, t_datum = {datum_len_ = 1, datum_typmod = 0, datum_typeid = 0}}, t_ctid = {ip_blkid = {bi_hi = 0, bi_lo = 1}, ip_posid = 40}, t_infomask2 = 30, t_infomask = 2313, t_hoff = 32 ' ', t_bits = 0x7f0002e52817 "\377\377\377\a"} (gdb)

(gdb) c Continuing. Breakpoint 1, HeapTupleSatisfiesMVCC (htup=0x1985d18, snapshot=0xf9bf60 <CatalogSnapshotData>, buffer=4) at tqual.c:966 966 HeapTupleHeader tuple = htup->t_data; (gdb) n 968 Assert(ItemPointerIsValid(&htup->t_self)); (gdb) 969 Assert(htup->t_tableOid != InvalidOid); (gdb) p *tuple $7 = {t_choice = {t_heap = {t_xmin = 2117, t_xmax = 0, t_field3 = {t_cid = 7, t_xvac = 7}}, t_datum = {datum_len_ = 2117, datum_typmod = 0, datum_typeid = 7}}, t_ctid = {ip_blkid = {bi_hi = 0, bi_lo = 1}, ip_posid = 58}, t_infomask2 = 33, t_infomask = 10505, t_hoff = 32 ' ', t_bits = 0x7f0002d25aa7 "\377\377\377?"} (gdb)

(gdb) n 971 if (!HeapTupleHeaderXminCommitted(tuple)) (gdb) 1073 if (!HeapTupleHeaderXminFrozen(tuple) && (gdb) 1074 XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot)) (gdb) 1073 if (!HeapTupleHeaderXminFrozen(tuple) && (gdb) 1080 if (tuple->t_infomask & HEAP_XMAX_INVALID) /* xid invalid or aborted */ (gdb) 1081 return true; (gdb) 1148 }

(gdb) c Continuing. Breakpoint 1, HeapTupleSatisfiesMVCC (htup=0x1985d18, snapshot=0x1a06358, buffer=44) at tqual.c:966 966 HeapTupleHeader tuple = htup->t_data; (gdb) n 968 Assert(ItemPointerIsValid(&htup->t_self)); (gdb) 969 Assert(htup->t_tableOid != InvalidOid); (gdb) p *tuple $8 = {t_choice = {t_heap = {t_xmin = 1, t_xmax = 0, t_field3 = {t_cid = 75, t_xvac = 75}}, t_datum = {datum_len_ = 1, datum_typmod = 0, datum_typeid = 75}}, t_ctid = {ip_blkid = {bi_hi = 0, bi_lo = 21}, ip_posid = 1}, t_infomask2 = 24, t_infomask = 2305, t_hoff = 32 ' ', t_bits = 0x7f0002d76307 "\377\377\017"} (gdb) ... (gdb) p *tuple $9 = {t_choice = {t_heap = {t_xmin = 1, t_xmax = 0, t_field3 = {t_cid = 75, t_xvac = 75}}, t_datum = {datum_len_ = 1, datum_typmod = 0, datum_typeid = 75}}, t_ctid = {ip_blkid = {bi_hi = 0, bi_lo = 1}, ip_posid = 19}, t_infomask2 = 20, t_infomask = 2307, t_hoff = 32 ' ', t_bits = 0x7f0002d7368f "\377\377\003"} ... (gdb) p *tuple $10 = {t_choice = {t_heap = {t_xmin = 1, t_xmax = 0, t_field3 = {t_cid = 0, t_xvac = 0}}, t_datum = {datum_len_ = 1, datum_typmod = 0, datum_typeid = 0}}, t_ctid = {ip_blkid = {bi_hi = 0, bi_lo = 0}, ip_posid = 1}, t_infomask2 = 4, t_infomask = 2313, t_hoff = 32 ' ', t_bits = 0x7f0002ee632f "\003"}

(gdb) c Continuing. Breakpoint 1, HeapTupleSatisfiesMVCC (htup=0x7ffc56ed8920, snapshot=0x1a062c0, buffer=206) at tqual.c:966 966 HeapTupleHeader tuple = htup->t_data; (gdb) n 968 Assert(ItemPointerIsValid(&htup->t_self)); (gdb) 969 Assert(htup->t_tableOid != InvalidOid); (gdb) p *tuple $11 = {t_choice = {t_heap = {t_xmin = 2360, t_xmax = 2362, t_field3 = {t_cid = 0, t_xvac = 0}}, t_datum = { datum_len_ = 2360, datum_typmod = 2362, datum_typeid = 0}}, t_ctid = {ip_blkid = {bi_hi = 0, bi_lo = 0}, ip_posid = 1}, t_infomask2 = 8195, t_infomask = 1282, t_hoff = 24 '\030', t_bits = 0x7f0002eba36f ""} (gdb)

1086 if (tuple->t_infomask & HEAP_XMAX_IS_MULTI) (gdb) 1113 if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED)) (gdb) 1141 if (XidInMVCCSnapshot(HeapTupleHeaderGetRawXmax(tuple), snapshot)) (gdb) n 1147 return false; (gdb)

(gdb) p *tuple $12 = {t_choice = {t_heap = {t_xmin = 2361, t_xmax = 2363, t_field3 = {t_cid = 0, t_xvac = 0}}, t_datum = { datum_len_ = 2361, datum_typmod = 2363, datum_typeid = 0}}, t_ctid = {ip_blkid = {bi_hi = 0, bi_lo = 0}, ip_posid = 3}, t_infomask2 = 16387, t_infomask = 258, t_hoff = 24 '\030', t_bits = 0x7f0002eba347 ""} (gdb)

1080 if (tuple->t_infomask & HEAP_XMAX_INVALID) /* xid invalid or aborted */ (gdb) 1083 if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask)) (gdb) 1086 if (tuple->t_infomask & HEAP_XMAX_IS_MULTI) (gdb) 1113 if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED)) (gdb) 1115 if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmax(tuple))) --> 非当前事务 (gdb) 1123 if (XidInMVCCSnapshot(HeapTupleHeaderGetRawXmax(tuple), snapshot)) --> 事务快照标志该事务进行中 (gdb) 1124 return true; --> 可见 (gdb)

(gdb) p *tuple $13 = {t_choice = {t_heap = {t_xmin = 2363, t_xmax = 0, t_field3 = {t_cid = 0, t_xvac = 0}}, t_datum = {datum_len_ = 2363, datum_typmod = 0, datum_typeid = 0}}, t_ctid = {ip_blkid = {bi_hi = 0, bi_lo = 0}, ip_posid = 3}, t_infomask2 = 32771, t_infomask = 10242, t_hoff = 24 '\030', t_bits = 0x7f0002eba31f ""} (gdb)

(gdb) n 971 if (!HeapTupleHeaderXminCommitted(tuple)) --> xmin未提交 (gdb) 973 if (HeapTupleHeaderXminInvalid(tuple)) (gdb) 977 if (tuple->t_infomask & HEAP_MOVED_OFF) (gdb) 996 else if (tuple->t_infomask & HEAP_MOVED_IN) (gdb) 1015 else if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tuple))) --> 非当前事务 (gdb) 1057 else if (XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot)) --> xmin事务处于活动中 (gdb) 1058 return false; --> 不可见 (gdb)

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一、数据结构

二、源码解读

三、跟踪分析

四、参考资料

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