Postgresql snapshot

http://www.chensj.cn/article/postgresql-source-code-analysis-big-picture/

PostGreSQL adopts "snapshot" method to realize MVCC.Specifically, this means that queries in each transaction can only see:

1. Data changes made by transactions that have been committed before the transaction was started.
2. Changes made by queries preceding this query in the current transaction.

PostGreSQL obtains a current database snapshot for each transaction when the transaction starts. The data structure in the snapshot is as follows:

typedef struct SnapshotData
{
SnapshotSatisfiesFunc satisfies; /*Row test function pointer*/

TransactionId xmin; /* All transaction changes with id less than xmin are visible in the current snapshot*/
TransactionId xmax;visible*/
uint32 xcnt; /* count of running transactions */
TransactionId *xip; /* id list of all running transactions */

/* note: all idsin xip[] satisfy xmin <= xip[i] < xmax */
int32 subxcnt; /* # of xact ids in subxip[], -1 if overflow */

TransactionId *subxip;* array of subxact IDs in progress */
commandId curcid; /* in my xact, CID < curcid are visible */
uint32 active_count; /* refcount on ActiveSnapshot/stack */
uint32 regd_count;refcount on RegisteredSnapshotList */
bool copied; /* false if it's a static snapshot */
} SnapshotData;


When a transaction is started, the snapshot is to record the visible TransactionID of the transaction, excluding the invisible ID.Each version of the data in PostGreSQL has two IDs, one of which is CreationID, which is the TransactionID that inserts the data, and the other is ExpiredID, which is the TransactionID that deletes or updates the data.The ID of the data visible to a Transaction must meet the following conditions:
      1.CreationID 2.ExpiredID>current TransactionID or ExpiredID does not exist

Transaction starts the process of forming a snapshot:
Traverse all currently active Transactions and record them in an active Transaction ID array; according to the transaction order, select the smallest TransactionID visible to the current Transaction, record it in xmin, select the largest visible TransactionID, and record it in xmax.