Обсуждение: what's hsitoric MVCC Snapshot?
Here are the comments in src/include/utils/snapshot.h.
/*
* For normal MVCC snapshot this contains the all xact IDs that are in
* progress, unless the snapshot was taken during recovery in which case
* it's empty. For historic MVCC snapshots, the meaning is inverted, i.e.
* it contains *committed* transactions between xmin and xmax.
*
* note: all ids in xip[] satisfy xmin <= xip[i] < xmax
*/
TransactionId *xip;
I can't understand the historic MVCC snapshots? can you give me a scenario
to describe this?
jacktby@gmail.com
On 3/5/23 06:49, jacktby@gmail.com wrote:
> Here are the comments in src/include/utils/snapshot.h.
> /*
> * For normal MVCC snapshot this contains the all xact IDs that are in
> * progress, unless the snapshot was taken during recovery in which case
> * it's empty. For historic MVCC snapshots, the meaning is inverted, i.e.
> * it contains *committed* transactions between xmin and xmax.
> *
> * note: all ids in xip[] satisfy xmin <= xip[i] < xmax
> */
> TransactionId *xip;
> I can't understand the historic MVCC snapshots? can you give me a scenario
> to describe this?
From the same file:
/*
* A tuple is visible iff it follows the rules of SNAPSHOT_MVCC, but
* supports being called in timetravel context (for decoding catalog
* contents in the context of logical decoding).
*/
SNAPSHOT_HISTORIC_MVCC,
> ------------------------------------------------------------------------
> jacktby@gmail.com
--
Adrian Klaver
adrian.klaver@aklaver.com
On 3/5/23 07:58, Adrian Klaver wrote: > On 3/5/23 06:49, jacktby@gmail.com wrote: >> Here are the comments in src/include/utils/snapshot.h. >> /* >> * For normal MVCC snapshot this contains the all xact IDs that are in >> * progress, unless the snapshot was taken during recovery in which case >> * it's empty. For historic MVCC snapshots, the meaning is inverted, i.e. >> * it contains *committed* transactions between xmin and xmax. >> * >> * note: all ids in xip[] satisfy xmin <= xip[i] < xmax >> */ >> TransactionId *xip; >> I can't understand the historic MVCC snapshots? can you give me >> a scenario >> to describe this? > > From the same file: > > > /* > * A tuple is visible iff it follows the rules of SNAPSHOT_MVCC, but > * supports being called in timetravel context (for decoding catalog > * contents in the context of logical decoding). > */ > SNAPSHOT_HISTORIC_MVCC, Also: typedef struct SnapshotData *Snapshot; #define InvalidSnapshot ((Snapshot) NULL) /* * Struct representing all kind of possible snapshots. * * There are several different kinds of snapshots: * * Normal MVCC snapshots * * MVCC snapshots taken during recovery (in Hot-Standby mode) * * Historic MVCC snapshots used during logical decoding * * snapshots passed to HeapTupleSatisfiesDirty() * * snapshots passed to HeapTupleSatisfiesNonVacuumable() * * snapshots used for SatisfiesAny, Toast, Self where no members are * accessed. * * TODO: It's probably a good idea to split this struct using a NodeTag * similar to how parser and executor nodes are handled, with one type for * each different kind of snapshot to avoid overloading the meaning of * individual fields. */ > >> ------------------------------------------------------------------------ >> jacktby@gmail.com > -- Adrian Klaver adrian.klaver@aklaver.com