1 /*-------------------------------------------------------------------------
4 * postgres transaction log interface routines
6 * Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
7 * Portions Copyright (c) 1994, Regents of the University of California
11 * src/backend/access/transam/transam.c
14 * This file contains the high level access-method interface to the
17 *-------------------------------------------------------------------------
22 #include "access/clog.h"
23 #include "access/subtrans.h"
24 #include "access/transam.h"
25 #include "utils/snapmgr.h"
28 * Single-item cache for results of TransactionLogFetch. It's worth having
29 * such a cache because we frequently find ourselves repeatedly checking the
30 * same XID, for example when scanning a table just after a bulk insert,
33 static TransactionId cachedFetchXid = InvalidTransactionId;
34 static XidStatus cachedFetchXidStatus;
35 static XLogRecPtr cachedCommitLSN;
38 static XidStatus TransactionLogFetch(TransactionId transactionId);
41 /* ----------------------------------------------------------------
42 * Postgres log access method interface
45 * ----------------------------------------------------------------
49 * TransactionLogFetch --- fetch commit status of specified transaction id
52 TransactionLogFetch(TransactionId transactionId)
58 * Before going to the commit log manager, check our single item cache to
59 * see if we didn't just check the transaction status a moment ago.
61 if (TransactionIdEquals(transactionId, cachedFetchXid))
62 return cachedFetchXidStatus;
65 * Also, check to see if the transaction ID is a permanent one.
67 if (!TransactionIdIsNormal(transactionId))
69 if (TransactionIdEquals(transactionId, BootstrapTransactionId))
70 return TRANSACTION_STATUS_COMMITTED;
71 if (TransactionIdEquals(transactionId, FrozenTransactionId))
72 return TRANSACTION_STATUS_COMMITTED;
73 return TRANSACTION_STATUS_ABORTED;
77 * Get the transaction status.
79 xidstatus = TransactionIdGetStatus(transactionId, &xidlsn);
82 * Cache it, but DO NOT cache status for unfinished or sub-committed
83 * transactions! We only cache status that is guaranteed not to change.
85 if (xidstatus != TRANSACTION_STATUS_IN_PROGRESS &&
86 xidstatus != TRANSACTION_STATUS_SUB_COMMITTED)
88 cachedFetchXid = transactionId;
89 cachedFetchXidStatus = xidstatus;
90 cachedCommitLSN = xidlsn;
96 /* ----------------------------------------------------------------
99 * TransactionIdDidCommit
100 * TransactionIdDidAbort
102 * these functions test the transaction status of
103 * a specified transaction id.
105 * TransactionIdCommitTree
106 * TransactionIdAsyncCommitTree
107 * TransactionIdAbortTree
109 * these functions set the transaction status of the specified
112 * See also TransactionIdIsInProgress, which once was in this module
113 * but now lives in procarray.c.
114 * ----------------------------------------------------------------
118 * TransactionIdDidCommit
119 * True iff transaction associated with the identifier did commit.
122 * Assumes transaction identifier is valid.
124 bool /* true if given transaction committed */
125 TransactionIdDidCommit(TransactionId transactionId)
129 xidstatus = TransactionLogFetch(transactionId);
132 * If it's marked committed, it's committed.
134 if (xidstatus == TRANSACTION_STATUS_COMMITTED)
138 * If it's marked subcommitted, we have to check the parent recursively.
139 * However, if it's older than TransactionXmin, we can't look at
140 * pg_subtrans; instead assume that the parent crashed without cleaning up
143 * Originally we Assert'ed that the result of SubTransGetParent was not
144 * zero. However with the introduction of prepared transactions, there can
145 * be a window just after database startup where we do not have complete
146 * knowledge in pg_subtrans of the transactions after TransactionXmin.
147 * StartupSUBTRANS() has ensured that any missing information will be
148 * zeroed. Since this case should not happen under normal conditions, it
149 * seems reasonable to emit a WARNING for it.
151 if (xidstatus == TRANSACTION_STATUS_SUB_COMMITTED)
153 TransactionId parentXid;
155 if (TransactionIdPrecedes(transactionId, TransactionXmin))
157 parentXid = SubTransGetParent(transactionId);
158 if (!TransactionIdIsValid(parentXid))
160 elog(WARNING, "no pg_subtrans entry for subcommitted XID %u",
164 return TransactionIdDidCommit(parentXid);
168 * It's not committed.
174 * TransactionIdDidAbort
175 * True iff transaction associated with the identifier did abort.
178 * Assumes transaction identifier is valid.
180 bool /* true if given transaction aborted */
181 TransactionIdDidAbort(TransactionId transactionId)
185 xidstatus = TransactionLogFetch(transactionId);
188 * If it's marked aborted, it's aborted.
190 if (xidstatus == TRANSACTION_STATUS_ABORTED)
194 * If it's marked subcommitted, we have to check the parent recursively.
195 * However, if it's older than TransactionXmin, we can't look at
196 * pg_subtrans; instead assume that the parent crashed without cleaning up
199 if (xidstatus == TRANSACTION_STATUS_SUB_COMMITTED)
201 TransactionId parentXid;
203 if (TransactionIdPrecedes(transactionId, TransactionXmin))
205 parentXid = SubTransGetParent(transactionId);
206 if (!TransactionIdIsValid(parentXid))
208 /* see notes in TransactionIdDidCommit */
209 elog(WARNING, "no pg_subtrans entry for subcommitted XID %u",
213 return TransactionIdDidAbort(parentXid);
223 * TransactionIdIsKnownCompleted
224 * True iff transaction associated with the identifier is currently
225 * known to have either committed or aborted.
227 * This does NOT look into pg_clog but merely probes our local cache
228 * (and so it's not named TransactionIdDidComplete, which would be the
229 * appropriate name for a function that worked that way). The intended
230 * use is just to short-circuit TransactionIdIsInProgress calls when doing
231 * repeated tqual.c checks for the same XID. If this isn't extremely fast
232 * then it will be counterproductive.
235 * Assumes transaction identifier is valid.
238 TransactionIdIsKnownCompleted(TransactionId transactionId)
240 if (TransactionIdEquals(transactionId, cachedFetchXid))
242 /* If it's in the cache at all, it must be completed. */
250 * TransactionIdCommitTree
251 * Marks the given transaction and children as committed
253 * "xid" is a toplevel transaction commit, and the xids array contains its
254 * committed subtransactions.
256 * This commit operation is not guaranteed to be atomic, but if not, subxids
257 * are correctly marked subcommit first.
260 TransactionIdCommitTree(TransactionId xid, int nxids, TransactionId *xids)
262 TransactionIdSetTreeStatus(xid, nxids, xids,
263 TRANSACTION_STATUS_COMMITTED,
268 * TransactionIdAsyncCommitTree
269 * Same as above, but for async commits. The commit record LSN is needed.
272 TransactionIdAsyncCommitTree(TransactionId xid, int nxids, TransactionId *xids,
275 TransactionIdSetTreeStatus(xid, nxids, xids,
276 TRANSACTION_STATUS_COMMITTED, lsn);
280 * TransactionIdAbortTree
281 * Marks the given transaction and children as aborted.
283 * "xid" is a toplevel transaction commit, and the xids array contains its
284 * committed subtransactions.
286 * We don't need to worry about the non-atomic behavior, since any onlookers
287 * will consider all the xacts as not-yet-committed anyway.
290 TransactionIdAbortTree(TransactionId xid, int nxids, TransactionId *xids)
292 TransactionIdSetTreeStatus(xid, nxids, xids,
293 TRANSACTION_STATUS_ABORTED, InvalidXLogRecPtr);
297 * TransactionIdPrecedes --- is id1 logically < id2?
300 TransactionIdPrecedes(TransactionId id1, TransactionId id2)
303 * If either ID is a permanent XID then we can just do unsigned
304 * comparison. If both are normal, do a modulo-2^32 comparison.
308 if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
311 diff = (int32) (id1 - id2);
316 * TransactionIdPrecedesOrEquals --- is id1 logically <= id2?
319 TransactionIdPrecedesOrEquals(TransactionId id1, TransactionId id2)
323 if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
326 diff = (int32) (id1 - id2);
331 * TransactionIdFollows --- is id1 logically > id2?
334 TransactionIdFollows(TransactionId id1, TransactionId id2)
338 if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
341 diff = (int32) (id1 - id2);
346 * TransactionIdFollowsOrEquals --- is id1 logically >= id2?
349 TransactionIdFollowsOrEquals(TransactionId id1, TransactionId id2)
353 if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
356 diff = (int32) (id1 - id2);
362 * TransactionIdLatest --- get latest XID among a main xact and its children
365 TransactionIdLatest(TransactionId mainxid,
366 int nxids, const TransactionId *xids)
368 TransactionId result;
371 * In practice it is highly likely that the xids[] array is sorted, and so
372 * we could save some cycles by just taking the last child XID, but this
373 * probably isn't so performance-critical that it's worth depending on
374 * that assumption. But just to show we're not totally stupid, scan the
375 * array back-to-front to avoid useless assignments.
380 if (TransactionIdPrecedes(result, xids[nxids]))
381 result = xids[nxids];
388 * TransactionIdGetCommitLSN
390 * This function returns an LSN that is late enough to be able
391 * to guarantee that if we flush up to the LSN returned then we
392 * will have flushed the transaction's commit record to disk.
394 * The result is not necessarily the exact LSN of the transaction's
395 * commit record! For example, for long-past transactions (those whose
396 * clog pages already migrated to disk), we'll return InvalidXLogRecPtr.
397 * Also, because we group transactions on the same clog page to conserve
398 * storage, we might return the LSN of a later transaction that falls into
402 TransactionIdGetCommitLSN(TransactionId xid)
407 * Currently, all uses of this function are for xids that were just
408 * reported to be committed by TransactionLogFetch, so we expect that
409 * checking TransactionLogFetch's cache will usually succeed and avoid an
410 * extra trip to shared memory.
412 if (TransactionIdEquals(xid, cachedFetchXid))
413 return cachedCommitLSN;
415 /* Special XIDs are always known committed */
416 if (!TransactionIdIsNormal(xid))
417 return InvalidXLogRecPtr;
420 * Get the transaction status.
422 (void) TransactionIdGetStatus(xid, &result);