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Commit973a210

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Tweak mdnblocks() to avoid doing lseek() on segments that it has
previously determined not to be the last segment of a relation.This reduces the expected cost to one seek, rather than one seek persegment. We can get away with this because truncation of a relationwill cause a relcache flush and so the md.c file descriptor will beclosed; when it is re-opened we will re-determine the last segment.
1 parentc9cf982 commit973a210

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  • src/backend/storage/smgr

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‎src/backend/storage/smgr/md.c

Lines changed: 16 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $Header: /cvsroot/pgsql/src/backend/storage/smgr/md.c,v 1.93 2002/11/12 15:26:30 tgl Exp $
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* $Header: /cvsroot/pgsql/src/backend/storage/smgr/md.c,v 1.94 2003/01/07 01:19:12 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
@@ -569,6 +569,21 @@ mdnblocks(Relation reln)
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#ifndefLET_OS_MANAGE_FILESIZE
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segno=0;
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/*
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* Skip through any segments that aren't the last one, to avoid redundant
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* seeks on them. We have previously verified that these segments are
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* exactly RELSEG_SIZE long, and it's useless to recheck that each time.
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* (NOTE: this assumption could only be wrong if another backend has
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* truncated the relation. We rely on higher code levels to handle that
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* scenario by closing and re-opening the md fd.)
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*/
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while (v->mdfd_chain!= (MdfdVec*)NULL)
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{
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segno++;
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v=v->mdfd_chain;
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}
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for (;;)
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{
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nblocks=_mdnblocks(v->mdfd_vfd,BLCKSZ);

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