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This section introduces the implementation logic of PostgreSQL for judging tuple visibility, focusing on the processing of the HeapTupleSatisfiesMVCC function.

I. data structure

Macro definition

/ / transaction ID, unsigned 32-bit integer typedef uint32 TransactionId;/*-* Special transaction ID values * transaction reserved by the system 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)) >

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