diff options
Diffstat (limited to 'src/backend/access/transam/clog.c')
| -rw-r--r-- | src/backend/access/transam/clog.c | 27 |
1 files changed, 19 insertions, 8 deletions
diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c index 69a81f32469..410d02a3943 100644 --- a/src/backend/access/transam/clog.c +++ b/src/backend/access/transam/clog.c @@ -694,6 +694,7 @@ CLOGShmemInit(void) SimpleLruInit(XactCtl, "Xact", CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE, XactSLRULock, "pg_xact", LWTRANCHE_XACT_BUFFER, SYNC_HANDLER_CLOG); + SlruPagePrecedesUnitTests(XactCtl, CLOG_XACTS_PER_PAGE); } /* @@ -912,13 +913,22 @@ TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid) /* - * Decide which of two CLOG page numbers is "older" for truncation purposes. + * Decide whether a CLOG page number is "older" for truncation purposes. * * We need to use comparison of TransactionIds here in order to do the right - * thing with wraparound XID arithmetic. However, if we are asked about - * page number zero, we don't want to hand InvalidTransactionId to - * TransactionIdPrecedes: it'll get weird about permanent xact IDs. So, - * offset both xids by FirstNormalTransactionId to avoid that. + * thing with wraparound XID arithmetic. However, TransactionIdPrecedes() + * would get weird about permanent xact IDs. So, offset both such that xid1, + * xid2, and xid2 + CLOG_XACTS_PER_PAGE - 1 are all normal XIDs; this offset + * is relevant to page 0 and to the page preceding page 0. + * + * The page containing oldestXact-2^31 is the important edge case. The + * portion of that page equaling or following oldestXact-2^31 is expendable, + * but the portion preceding oldestXact-2^31 is not. When oldestXact-2^31 is + * the first XID of a page and segment, the entire page and segment is + * expendable, and we could truncate the segment. Recognizing that case would + * require making oldestXact, not just the page containing oldestXact, + * available to this callback. The benefit would be rare and small, so we + * don't optimize that edge case. */ static bool CLOGPagePrecedes(int page1, int page2) @@ -927,11 +937,12 @@ CLOGPagePrecedes(int page1, int page2) TransactionId xid2; xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE; - xid1 += FirstNormalTransactionId; + xid1 += FirstNormalTransactionId + 1; xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE; - xid2 += FirstNormalTransactionId; + xid2 += FirstNormalTransactionId + 1; - return TransactionIdPrecedes(xid1, xid2); + return (TransactionIdPrecedes(xid1, xid2) && + TransactionIdPrecedes(xid1, xid2 + CLOG_XACTS_PER_PAGE - 1)); } |