Avoid changing an index's indcheckxmin horizon during REINDEX.

There can never be a need to push the indcheckxmin horizon forward, since
any HOT chains that are actually broken with respect to the index must
pre-date its original creation.  So we can just avoid changing pg_index
altogether during a REINDEX operation.

This offers a cleaner solution than my previous patch for the problem
found a few days ago that we mustn't try to update pg_index while we are
reindexing it.  System catalog indexes will always be created with
indcheckxmin = false during initdb, and with this modified code we should
never try to change their pg_index entries.  This avoids special-casing
system catalogs as the former patch did, and should provide a performance
benefit for many cases where REINDEX formerly caused an index to be
considered unusable for a short time.

Back-patch to 8.3 to cover all versions containing HOT.  Note that this
patch changes the API for index_build(), but I believe it is unlikely that
any add-on code is calling that directly.

diff --git a/src/backend/bootstrap/bootstrap.c b/src/backend/bootstrap/bootstrap.c
index 5a0f852b6fef0a40769ae430b2bf38edd9cd60b5..faf5582edfcfa496a77ab40956d06679db7af7ab 100644 (file)
--- a/src/backend/bootstrap/bootstrap.c
+++ b/src/backend/bootstrap/bootstrap.c
@@ -1286,7 +1286,7 @@ build_indices(void)
                heap = heap_open(ILHead->il_heap, NoLock);
                ind = index_open(ILHead->il_ind, NoLock);
 
-               index_build(heap, ind, ILHead->il_info, false);
+               index_build(heap, ind, ILHead->il_info, false, false);
 
                index_close(ind, NoLock);
                heap_close(heap, NoLock);

diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c
index 071266536df2683b03334663dcb510e82c5cf191..70a32011f428c32cb93031683c57dff5a2a41df7 100644 (file)
--- a/src/backend/catalog/heap.c
+++ b/src/backend/catalog/heap.c
@@ -2344,7 +2344,7 @@ RelationTruncateIndexes(Relation heapRelation)
 
                /* Initialize the index and rebuild */
                /* Note: we do not need to re-establish pkey setting */
-               index_build(heapRelation, currentIndex, indexInfo, false);
+               index_build(heapRelation, currentIndex, indexInfo, false, true);
 
                /* We're done with this index */
                index_close(currentIndex, NoLock);

diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index 487dbbf24d247251d3078839009687f7c96af7bc..a074150b63026384a14b5f4995401b0d10e1e9a1 100644 (file)
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -866,7 +866,7 @@ index_create(Oid heapRelationId,
        }
        else
        {
-               index_build(heapRelation, indexRelation, indexInfo, isprimary);
+               index_build(heapRelation, indexRelation, indexInfo, isprimary, false);
        }
 
        /*
@@ -1357,8 +1357,11 @@ setNewRelfilenode(Relation relation, TransactionId freezeXid)
  * entries of the index and heap relation as needed, using statistics
  * returned by ambuild as well as data passed by the caller.
  *
- * Note: when reindexing an existing index, isprimary can be false;
- * the index is already properly marked and need not be re-marked.
+ * isprimary tells whether to mark the index as a primary-key index.
+ * isreindex indicates we are recreating a previously-existing index.
+ *
+ * Note: when reindexing an existing index, isprimary can be false even if
+ * the index is a PK; it's already properly marked and need not be re-marked.
  *
  * Note: before Postgres 8.2, the passed-in heap and index Relations
  * were automatically closed by this routine.  This is no longer the case.
@@ -1368,7 +1371,8 @@ void
 index_build(Relation heapRelation,
                        Relation indexRelation,
                        IndexInfo *indexInfo,
-                       bool isprimary)
+                       bool isprimary,
+                       bool isreindex)
 {
        RegProcedure procedure;
        IndexBuildResult *stats;
@@ -1415,8 +1419,15 @@ index_build(Relation heapRelation,
         * If we found any potentially broken HOT chains, mark the index as not
         * being usable until the current transaction is below the event horizon.
         * See src/backend/access/heap/README.HOT for discussion.
-        */
-       if (indexInfo->ii_BrokenHotChain)
+        *
+        * However, when reindexing an existing index, we should do nothing here.
+        * Any HOT chains that are broken with respect to the index must predate
+        * the index's original creation, so there is no need to change the
+        * index's usability horizon.  Moreover, we *must not* try to change
+        * the index's pg_index entry while reindexing pg_index itself, and this
+        * optimization nicely prevents that.
+        */
+       if (indexInfo->ii_BrokenHotChain && !isreindex)
        {
                Oid                     indexId = RelationGetRelid(indexRelation);
                Relation        pg_index;
@@ -1432,6 +1443,9 @@ index_build(Relation heapRelation,
                        elog(ERROR, "cache lookup failed for index %u", indexId);
                indexForm = (Form_pg_index) GETSTRUCT(indexTuple);
 
+               /* If it's a new index, indcheckxmin shouldn't be set ... */
+               Assert(!indexForm->indcheckxmin);
+
                indexForm->indcheckxmin = true;
                simple_heap_update(pg_index, &indexTuple->t_self, indexTuple);
                CatalogUpdateIndexes(pg_index, indexTuple);
@@ -2333,7 +2347,7 @@ reindex_index(Oid indexId)
 
                /* Initialize the index and rebuild */
                /* Note: we do not need to re-establish pkey setting */
-               index_build(heapRelation, iRel, indexInfo, false);
+               index_build(heapRelation, iRel, indexInfo, false, true);
        }
        PG_CATCH();
        {
@@ -2352,6 +2366,16 @@ reindex_index(Oid indexId)
         * We can also reset indcheckxmin, because we have now done a
         * non-concurrent index build, *except* in the case where index_build
         * found some still-broken HOT chains.
+        *
+        * Note that it is important to not update the pg_index entry if we don't
+        * have to, because updating it will move the index's usability horizon
+        * (recorded as the tuple's xmin value) if indcheckxmin is true.  We don't
+        * really want REINDEX to move the usability horizon forward ever, but we
+        * have no choice if we are to fix indisvalid or indisready.  Of course,
+        * clearing indcheckxmin eliminates the issue, so we're happy to do that
+        * if we can.  Another reason for caution here is that while reindexing
+        * pg_index itself, we must not try to update it.  We assume that
+        * pg_index's indexes will always have these flags in their clean state.
         */
        pg_index = heap_open(IndexRelationId, RowExclusiveLock);
 

diff --git a/src/backend/commands/cluster.c b/src/backend/commands/cluster.c
index 816cf47bb3d35e4c7b94c0b35153789ebff710cf..f6dc6e2c017e7dd52edddab31205af2e3b168bf8 100644 (file)
--- a/src/backend/commands/cluster.c
+++ b/src/backend/commands/cluster.c
@@ -626,6 +626,12 @@ rebuild_relation(Relation OldHeap, Oid indexOid)
         * Rebuild each index on the relation (but not the toast table, which is
         * all-new at this point).      We do not need CommandCounterIncrement()
         * because reindex_relation does it.
+        *
+        * Note: because index_build is called via reindex_relation, it will never
+        * set indcheckxmin true for the indexes.  This is OK even though in some
+        * sense we are building new indexes rather than rebuilding existing ones,
+        * because the new heap won't contain any HOT chains at all, let alone
+        * broken ones, so it can't be necessary to set indcheckxmin.
         */
        reindex_relation(tableOid, false);
 

diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 77d6e53f70bc2a7a522235f1c5afff8bc150687d..6ba5b5d36b83e77cf07862015509eaa3e3796d25 100644 (file)
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -554,7 +554,7 @@ DefineIndex(RangeVar *heapRelation,
        indexInfo->ii_BrokenHotChain = false;
 
        /* Now build the index */
-       index_build(rel, indexRelation, indexInfo, primary);
+       index_build(rel, indexRelation, indexInfo, primary, false);
 
        /* Close both the relations, but keep the locks */
        heap_close(rel, NoLock);

diff --git a/src/include/catalog/index.h b/src/include/catalog/index.h
index 18064889aa10ee9dfb05684d602ca883894885f1..5eca12b2ef563a907d6a89c2418407463861accb 100644 (file)
--- a/src/include/catalog/index.h
+++ b/src/include/catalog/index.h
@@ -58,7 +58,8 @@ extern void setNewRelfilenode(Relation relation, TransactionId freezeXid);
 extern void index_build(Relation heapRelation,
                        Relation indexRelation,
                        IndexInfo *indexInfo,
-                       bool isprimary);
+                       bool isprimary,
+                       bool isreindex);
 
 extern double IndexBuildHeapScan(Relation heapRelation,
                                   Relation indexRelation,