And here is a version for 9.1.  This omits the code changes directly
relevant to DROP INDEX CONCURRENTLY, but includes the changes to avoid
transactional updates of the pg_index row during CREATE CONCURRENTLY,
as well as the changes to prevent use of not-valid or not-ready indexes
in places where it matters.  I also chose to keep on using the
IndexIsValid and IndexIsReady macros, so as to avoid unnecessary
divergences of the branches.

I think this much of the patch needs to go into all supported branches.

                        regards, tom lane

diff --git a/src/backend/access/heap/README.HOT b/src/backend/access/heap/README.HOT
index f12cad44e56c363e62fa617497bbedfe1ba8c1fe..7cbc6a1d7e7328364938ef5d69bebe865524d7f8 100644
*** a/src/backend/access/heap/README.HOT
--- b/src/backend/access/heap/README.HOT
*************** from the index, as well as ensuring that
*** 386,391 ****
--- 386,397 ----
  rows in a broken HOT chain (the first condition is stronger than the
  second).  Finally, we can mark the index valid for searches.
  
+ Note that we do not need to set pg_index.indcheckxmin in this code path,
+ because we have outwaited any transactions that would need to avoid using
+ the index.  (indcheckxmin is only needed because non-concurrent CREATE
+ INDEX doesn't want to wait; its stronger lock would create too much risk of
+ deadlock if it did.)
+ 
  
  Limitations and Restrictions
  ----------------------------
diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index 2e023d5ab56ac4efe1ab243739b48149e67a7408..129a1ac11f0e79408961abd73f0a33395569c5d5 100644
*** a/src/backend/catalog/index.c
--- b/src/backend/catalog/index.c
*************** static void ResetReindexPending(void);
*** 129,134 ****
--- 129,138 ----
   *		See whether an existing relation has a primary key.
   *
   * Caller must have suitable lock on the relation.
+  *
+  * Note: we intentionally do not check IndexIsValid here; that's because this
+  * is used to enforce the rule that there can be only one indisprimary index,
+  * and we want that to be true even if said index is invalid.
   */
  static bool
  relationHasPrimaryKey(Relation rel)
*************** index_constraint_create(Relation heapRel
*** 1247,1254 ****
  	 * Note: since this is a transactional update, it's unsafe against
  	 * concurrent SnapshotNow scans of pg_index.  When making an existing
  	 * index into a constraint, caller must have a table lock that prevents
! 	 * concurrent table updates, and there is a risk that concurrent readers
! 	 * of the table will miss seeing this index at all.
  	 */
  	if (update_pgindex && (mark_as_primary || deferrable))
  	{
--- 1251,1259 ----
  	 * Note: since this is a transactional update, it's unsafe against
  	 * concurrent SnapshotNow scans of pg_index.  When making an existing
  	 * index into a constraint, caller must have a table lock that prevents
! 	 * concurrent table updates; if it's less than a full exclusive lock,
! 	 * there is a risk that concurrent readers of the table will miss seeing
! 	 * this index at all.
  	 */
  	if (update_pgindex && (mark_as_primary || deferrable))
  	{
*************** BuildIndexInfo(Relation index)
*** 1450,1456 ****
  
  	/* other info */
  	ii->ii_Unique = indexStruct->indisunique;
! 	ii->ii_ReadyForInserts = indexStruct->indisready;
  
  	/* initialize index-build state to default */
  	ii->ii_Concurrent = false;
--- 1455,1461 ----
  
  	/* other info */
  	ii->ii_Unique = indexStruct->indisunique;
! 	ii->ii_ReadyForInserts = IndexIsReady(indexStruct);
  
  	/* initialize index-build state to default */
  	ii->ii_Concurrent = false;
*************** index_build(Relation heapRelation,
*** 1789,1796 ****
  	 * 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;
--- 1794,1813 ----
  	 * 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.
+ 	 *
+ 	 * We also need not set indcheckxmin during a concurrent index build,
+ 	 * because we won't set indisvalid true until all transactions that care
+ 	 * about the broken HOT chains are gone.
+ 	 *
+ 	 * Therefore, this code path can only be taken during non-concurrent
+ 	 * CREATE INDEX.  Thus the fact that heap_update will set the pg_index
+ 	 * tuple's xmin doesn't matter, because that tuple was created in the
+ 	 * current transaction anyway.	That also means we don't need to worry
+ 	 * about any concurrent readers of the tuple; no other transaction can see
+ 	 * it yet.
  	 */
! 	if (indexInfo->ii_BrokenHotChain && !isreindex &&
! 		!indexInfo->ii_Concurrent)
  	{
  		Oid			indexId = RelationGetRelid(indexRelation);
  		Relation	pg_index;
*************** validate_index_heapscan(Relation heapRel
*** 2754,2759 ****
--- 2771,2835 ----
  
  
  /*
+  * index_set_state_flags - adjust pg_index state flags
+  *
+  * This is used during CREATE INDEX CONCURRENTLY to adjust the pg_index
+  * flags that denote the index's state.  We must use an in-place update of
+  * the pg_index tuple, because we do not have exclusive lock on the parent
+  * table and so other sessions might concurrently be doing SnapshotNow scans
+  * of pg_index to identify the table's indexes.  A transactional update would
+  * risk somebody not seeing the index at all.  Because the update is not
+  * transactional and will not roll back on error, this must only be used as
+  * the last step in a transaction that has not made any transactional catalog
+  * updates!
+  *
+  * Note that heap_inplace_update does send a cache inval message for the
+  * tuple, so other sessions will hear about the update as soon as we commit.
+  */
+ void
+ index_set_state_flags(Oid indexId, IndexStateFlagsAction action)
+ {
+ 	Relation	pg_index;
+ 	HeapTuple	indexTuple;
+ 	Form_pg_index indexForm;
+ 
+ 	/* Assert that current xact hasn't done any transactional updates */
+ 	Assert(GetTopTransactionIdIfAny() == InvalidTransactionId);
+ 
+ 	/* Open pg_index and fetch a writable copy of the index's tuple */
+ 	pg_index = heap_open(IndexRelationId, RowExclusiveLock);
+ 
+ 	indexTuple = SearchSysCacheCopy1(INDEXRELID,
+ 									 ObjectIdGetDatum(indexId));
+ 	if (!HeapTupleIsValid(indexTuple))
+ 		elog(ERROR, "cache lookup failed for index %u", indexId);
+ 	indexForm = (Form_pg_index) GETSTRUCT(indexTuple);
+ 
+ 	/* Perform the requested state change on the copy */
+ 	switch (action)
+ 	{
+ 		case INDEX_CREATE_SET_READY:
+ 			/* Set indisready during a CREATE INDEX CONCURRENTLY sequence */
+ 			Assert(!indexForm->indisready);
+ 			Assert(!indexForm->indisvalid);
+ 			indexForm->indisready = true;
+ 			break;
+ 		case INDEX_CREATE_SET_VALID:
+ 			/* Set indisvalid during a CREATE INDEX CONCURRENTLY sequence */
+ 			Assert(indexForm->indisready);
+ 			Assert(!indexForm->indisvalid);
+ 			indexForm->indisvalid = true;
+ 			break;
+ 	}
+ 
+ 	/* ... and write it back in-place */
+ 	heap_inplace_update(pg_index, indexTuple);
+ 
+ 	heap_close(pg_index, RowExclusiveLock);
+ }
+ 
+ 
+ /*
   * IndexGetRelation: given an index's relation OID, get the OID of the
   * relation it is an index on.	Uses the system cache.
   */
*************** void
*** 2782,2793 ****
  reindex_index(Oid indexId, bool skip_constraint_checks)
  {
  	Relation	iRel,
! 				heapRelation,
! 				pg_index;
  	Oid			heapId;
  	IndexInfo  *indexInfo;
- 	HeapTuple	indexTuple;
- 	Form_pg_index indexForm;
  	volatile bool skipped_constraint = false;
  
  	/*
--- 2858,2866 ----
  reindex_index(Oid indexId, bool skip_constraint_checks)
  {
  	Relation	iRel,
! 				heapRelation;
  	Oid			heapId;
  	IndexInfo  *indexInfo;
  	volatile bool skipped_constraint = false;
  
  	/*
*************** reindex_index(Oid indexId, bool skip_con
*** 2867,2891 ****
  	 *
  	 * 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.	If it did, we normally leave
! 	 * indcheckxmin alone (note that index_build won't have changed it,
! 	 * because this is a reindex).	But if the index was invalid or not ready
! 	 * and there were broken HOT chains, it seems best to force indcheckxmin
! 	 * true, because the normal argument that the HOT chains couldn't conflict
! 	 * with the index is suspect for an invalid index.
  	 *
! 	 * 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.
  	 */
  	if (!skipped_constraint)
  	{
  		pg_index = heap_open(IndexRelationId, RowExclusiveLock);
  
  		indexTuple = SearchSysCacheCopy1(INDEXRELID,
--- 2940,2978 ----
  	 *
  	 * 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. If it did, and we don't have to
! 	 * change any of the other flags, we just leave indcheckxmin alone (note
! 	 * that index_build won't have changed it, because this is a reindex).
! 	 * This is okay and desirable because not updating the tuple leaves the
! 	 * index's usability horizon (recorded as the tuple's xmin value) the same
! 	 * as it was.
  	 *
! 	 * But, if the index was invalid/not-ready and there were broken HOT
! 	 * chains, we had better force indcheckxmin true, because the normal
! 	 * argument that the HOT chains couldn't conflict with the index is
! 	 * suspect for an invalid index.  In this case advancing the usability
! 	 * horizon is appropriate.
! 	 *
! 	 * Note that if we have to update the tuple, there is a risk of concurrent
! 	 * transactions not seeing it during their SnapshotNow scans of pg_index.
! 	 * While not especially desirable, this is safe because no such
! 	 * transaction could be trying to update the table (since we have
! 	 * ShareLock on it).  The worst case is that someone might transiently
! 	 * fail to use the index for a query --- but it was probably unusable
! 	 * before anyway, if we are updating the tuple.
! 	 *
! 	 * Another reason for avoiding unnecessary updates here is that while
! 	 * reindexing pg_index itself, we must not try to update tuples in it.
! 	 * pg_index's indexes should always have these flags in their clean state,
! 	 * so that won't happen.
  	 */
  	if (!skipped_constraint)
  	{
+ 		Relation	pg_index;
+ 		HeapTuple	indexTuple;
+ 		Form_pg_index indexForm;
+ 		bool		index_bad;
+ 
  		pg_index = heap_open(IndexRelationId, RowExclusiveLock);
  
  		indexTuple = SearchSysCacheCopy1(INDEXRELID,
*************** reindex_index(Oid indexId, bool skip_con
*** 2894,2910 ****
  			elog(ERROR, "cache lookup failed for index %u", indexId);
  		indexForm = (Form_pg_index) GETSTRUCT(indexTuple);
  
! 		if (!indexForm->indisvalid || !indexForm->indisready ||
  			(indexForm->indcheckxmin && !indexInfo->ii_BrokenHotChain))
  		{
  			if (!indexInfo->ii_BrokenHotChain)
  				indexForm->indcheckxmin = false;
! 			else if (!indexForm->indisvalid || !indexForm->indisready)
  				indexForm->indcheckxmin = true;
  			indexForm->indisvalid = true;
  			indexForm->indisready = true;
  			simple_heap_update(pg_index, &indexTuple->t_self, indexTuple);
  			CatalogUpdateIndexes(pg_index, indexTuple);
  		}
  
  		heap_close(pg_index, RowExclusiveLock);
--- 2981,3008 ----
  			elog(ERROR, "cache lookup failed for index %u", indexId);
  		indexForm = (Form_pg_index) GETSTRUCT(indexTuple);
  
! 		index_bad = (!indexForm->indisvalid ||
! 					 !indexForm->indisready);
! 		if (index_bad ||
  			(indexForm->indcheckxmin && !indexInfo->ii_BrokenHotChain))
  		{
  			if (!indexInfo->ii_BrokenHotChain)
  				indexForm->indcheckxmin = false;
! 			else if (index_bad)
  				indexForm->indcheckxmin = true;
  			indexForm->indisvalid = true;
  			indexForm->indisready = true;
  			simple_heap_update(pg_index, &indexTuple->t_self, indexTuple);
  			CatalogUpdateIndexes(pg_index, indexTuple);
+ 
+ 			/*
+ 			 * Invalidate the relcache for the table, so that after we commit
+ 			 * all sessions will refresh the table's index list.  This ensures
+ 			 * that if anyone misses seeing the pg_index row during this
+ 			 * update, they'll refresh their list before attempting any update
+ 			 * on the table.
+ 			 */
+ 			CacheInvalidateRelcache(heapRelation);
  		}
  
  		heap_close(pg_index, RowExclusiveLock);
diff --git a/src/backend/commands/cluster.c b/src/backend/commands/cluster.c
index 5dec4298e2c735438861739ed0b3adc87e1eea6d..81e72825269220e66488c93fef7b26411fe58f9b 100644
*** a/src/backend/commands/cluster.c
--- b/src/backend/commands/cluster.c
*************** check_index_is_clusterable(Relation OldH
*** 454,460 ****
  	 * might put recently-dead tuples out-of-order in the new table, and there
  	 * is little harm in that.)
  	 */
! 	if (!OldIndex->rd_index->indisvalid)
  		ereport(ERROR,
  				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  				 errmsg("cannot cluster on invalid index \"%s\"",
--- 454,460 ----
  	 * might put recently-dead tuples out-of-order in the new table, and there
  	 * is little harm in that.)
  	 */
! 	if (!IndexIsValid(OldIndex->rd_index))
  		ereport(ERROR,
  				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  				 errmsg("cannot cluster on invalid index \"%s\"",
*************** check_index_is_clusterable(Relation OldH
*** 468,473 ****
--- 468,478 ----
   * mark_index_clustered: mark the specified index as the one clustered on
   *
   * With indexOid == InvalidOid, will mark all indexes of rel not-clustered.
+  *
+  * Note: we do transactional updates of the pg_index rows, which are unsafe
+  * against concurrent SnapshotNow scans of pg_index.  Therefore this is unsafe
+  * to execute with less than full exclusive lock on the parent table;
+  * otherwise concurrent executions of RelationGetIndexList could miss indexes.
   */
  void
  mark_index_clustered(Relation rel, Oid indexOid)
*************** mark_index_clustered(Relation rel, Oid i
*** 523,528 ****
--- 528,536 ----
  		}
  		else if (thisIndexOid == indexOid)
  		{
+ 			/* this was checked earlier, but let's be real sure */
+ 			if (!IndexIsValid(indexForm))
+ 				elog(ERROR, "cannot cluster on invalid index %u", indexOid);
  			indexForm->indisclustered = true;
  			simple_heap_update(pg_index, &indexTuple->t_self, indexTuple);
  			CatalogUpdateIndexes(pg_index, indexTuple);
diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index b7c021d943a99e96ff5aab93caff7aecc83df66f..a08ad1324ba34e9c06d91cad50c4beb3d345892e 100644
*** a/src/backend/commands/indexcmds.c
--- b/src/backend/commands/indexcmds.c
*************** DefineIndex(RangeVar *heapRelation,
*** 148,156 ****
  	LockRelId	heaprelid;
  	LOCKTAG		heaplocktag;
  	Snapshot	snapshot;
- 	Relation	pg_index;
- 	HeapTuple	indexTuple;
- 	Form_pg_index indexForm;
  	int			i;
  
  	/*
--- 148,153 ----
*************** DefineIndex(RangeVar *heapRelation,
*** 531,553 ****
  	 * commit this transaction, any new transactions that open the table must
  	 * insert new entries into the index for insertions and non-HOT updates.
  	 */
! 	pg_index = heap_open(IndexRelationId, RowExclusiveLock);
! 
! 	indexTuple = SearchSysCacheCopy1(INDEXRELID,
! 									 ObjectIdGetDatum(indexRelationId));
! 	if (!HeapTupleIsValid(indexTuple))
! 		elog(ERROR, "cache lookup failed for index %u", indexRelationId);
! 	indexForm = (Form_pg_index) GETSTRUCT(indexTuple);
! 
! 	Assert(!indexForm->indisready);
! 	Assert(!indexForm->indisvalid);
! 
! 	indexForm->indisready = true;
! 
! 	simple_heap_update(pg_index, &indexTuple->t_self, indexTuple);
! 	CatalogUpdateIndexes(pg_index, indexTuple);
! 
! 	heap_close(pg_index, RowExclusiveLock);
  
  	/* we can do away with our snapshot */
  	PopActiveSnapshot();
--- 528,534 ----
  	 * commit this transaction, any new transactions that open the table must
  	 * insert new entries into the index for insertions and non-HOT updates.
  	 */
! 	index_set_state_flags(indexRelationId, INDEX_CREATE_SET_READY);
  
  	/* we can do away with our snapshot */
  	PopActiveSnapshot();
*************** DefineIndex(RangeVar *heapRelation,
*** 671,693 ****
  	/*
  	 * Index can now be marked valid -- update its pg_index entry
  	 */
! 	pg_index = heap_open(IndexRelationId, RowExclusiveLock);
! 
! 	indexTuple = SearchSysCacheCopy1(INDEXRELID,
! 									 ObjectIdGetDatum(indexRelationId));
! 	if (!HeapTupleIsValid(indexTuple))
! 		elog(ERROR, "cache lookup failed for index %u", indexRelationId);
! 	indexForm = (Form_pg_index) GETSTRUCT(indexTuple);
! 
! 	Assert(indexForm->indisready);
! 	Assert(!indexForm->indisvalid);
! 
! 	indexForm->indisvalid = true;
! 
! 	simple_heap_update(pg_index, &indexTuple->t_self, indexTuple);
! 	CatalogUpdateIndexes(pg_index, indexTuple);
! 
! 	heap_close(pg_index, RowExclusiveLock);
  
  	/*
  	 * The pg_index update will cause backends (including this one) to update
--- 652,658 ----
  	/*
  	 * Index can now be marked valid -- update its pg_index entry
  	 */
! 	index_set_state_flags(indexRelationId, INDEX_CREATE_SET_VALID);
  
  	/*
  	 * The pg_index update will cause backends (including this one) to update
*************** DefineIndex(RangeVar *heapRelation,
*** 695,701 ****
  	 * relcache inval on the parent table to force replanning of cached plans.
  	 * Otherwise existing sessions might fail to use the new index where it
  	 * would be useful.  (Note that our earlier commits did not create reasons
! 	 * to replan; relcache flush on the index itself was sufficient.)
  	 */
  	CacheInvalidateRelcacheByRelid(heaprelid.relId);
  
--- 660,666 ----
  	 * relcache inval on the parent table to force replanning of cached plans.
  	 * Otherwise existing sessions might fail to use the new index where it
  	 * would be useful.  (Note that our earlier commits did not create reasons
! 	 * to replan; so relcache flush on the index itself was sufficient.)
  	 */
  	CacheInvalidateRelcacheByRelid(heaprelid.relId);
  
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 99fb892c1941955d1ae6acc15d68a5a5a50ad641..d7bb20fa2ec17aba06b61b24d31185fd06eecc0f 100644
*** a/src/backend/commands/tablecmds.c
--- b/src/backend/commands/tablecmds.c
*************** ATExecDropNotNull(Relation rel, const ch
*** 4528,4533 ****
--- 4528,4535 ----
  
  	/*
  	 * Check that the attribute is not in a primary key
+ 	 *
+ 	 * Note: we'll throw error even if the pkey index is not valid.
  	 */
  
  	/* Loop over all indexes on the relation */
*************** transformFkeyGetPrimaryKey(Relation pkre
*** 5876,5882 ****
  	/*
  	 * Get the list of index OIDs for the table from the relcache, and look up
  	 * each one in the pg_index syscache until we find one marked primary key
! 	 * (hopefully there isn't more than one such).
  	 */
  	*indexOid = InvalidOid;
  
--- 5878,5884 ----
  	/*
  	 * Get the list of index OIDs for the table from the relcache, and look up
  	 * each one in the pg_index syscache until we find one marked primary key
! 	 * (hopefully there isn't more than one such).  Insist it's valid, too.
  	 */
  	*indexOid = InvalidOid;
  
*************** transformFkeyGetPrimaryKey(Relation pkre
*** 5890,5896 ****
  		if (!HeapTupleIsValid(indexTuple))
  			elog(ERROR, "cache lookup failed for index %u", indexoid);
  		indexStruct = (Form_pg_index) GETSTRUCT(indexTuple);
! 		if (indexStruct->indisprimary)
  		{
  			/*
  			 * Refuse to use a deferrable primary key.	This is per SQL spec,
--- 5892,5898 ----
  		if (!HeapTupleIsValid(indexTuple))
  			elog(ERROR, "cache lookup failed for index %u", indexoid);
  		indexStruct = (Form_pg_index) GETSTRUCT(indexTuple);
! 		if (indexStruct->indisprimary && IndexIsValid(indexStruct))
  		{
  			/*
  			 * Refuse to use a deferrable primary key.	This is per SQL spec,
*************** transformFkeyCheckAttrs(Relation pkrel,
*** 5988,5997 ****
  
  		/*
  		 * Must have the right number of columns; must be unique and not a
! 		 * partial index; forget it if there are any expressions, too
  		 */
  		if (indexStruct->indnatts == numattrs &&
  			indexStruct->indisunique &&
  			heap_attisnull(indexTuple, Anum_pg_index_indpred) &&
  			heap_attisnull(indexTuple, Anum_pg_index_indexprs))
  		{
--- 5990,6001 ----
  
  		/*
  		 * Must have the right number of columns; must be unique and not a
! 		 * partial index; forget it if there are any expressions, too. Invalid
! 		 * indexes are out as well.
  		 */
  		if (indexStruct->indnatts == numattrs &&
  			indexStruct->indisunique &&
+ 			IndexIsValid(indexStruct) &&
  			heap_attisnull(indexTuple, Anum_pg_index_indpred) &&
  			heap_attisnull(indexTuple, Anum_pg_index_indexprs))
  		{
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index fdd9fb4336ec52828adb5e92c17a5fa10aaa2652..bfdf8e38b1af6edb03e64c4dc50fb7e1638e8bf0 100644
*** a/src/backend/commands/vacuum.c
--- b/src/backend/commands/vacuum.c
*************** vacuum_rel(Oid relid, VacuumStmt *vacstm
*** 1078,1086 ****
  
  
  /*
!  * Open all the indexes of the given relation, obtaining the specified kind
!  * of lock on each.  Return an array of Relation pointers for the indexes
!  * into *Irel, and the number of indexes into *nindexes.
   */
  void
  vac_open_indexes(Relation relation, LOCKMODE lockmode,
--- 1078,1093 ----
  
  
  /*
!  * Open all the vacuumable indexes of the given relation, obtaining the
!  * specified kind of lock on each.	Return an array of Relation pointers for
!  * the indexes into *Irel, and the number of indexes into *nindexes.
!  *
!  * We consider an index vacuumable if it is marked insertable (IndexIsReady).
!  * If it isn't, probably a CREATE INDEX CONCURRENTLY command failed early in
!  * execution, and what we have is too corrupt to be processable.  We will
!  * vacuum even if the index isn't indisvalid; this is important because in a
!  * unique index, uniqueness checks will be performed anyway and had better not
!  * hit dangling index pointers.
   */
  void
  vac_open_indexes(Relation relation, LOCKMODE lockmode,
*************** vac_open_indexes(Relation relation, LOCK
*** 1094,1114 ****
  
  	indexoidlist = RelationGetIndexList(relation);
  
! 	*nindexes = list_length(indexoidlist);
  
! 	if (*nindexes > 0)
! 		*Irel = (Relation *) palloc(*nindexes * sizeof(Relation));
  	else
  		*Irel = NULL;
  
  	i = 0;
  	foreach(indexoidscan, indexoidlist)
  	{
  		Oid			indexoid = lfirst_oid(indexoidscan);
  
! 		(*Irel)[i++] = index_open(indexoid, lockmode);
  	}
  
  	list_free(indexoidlist);
  }
  
--- 1101,1130 ----
  
  	indexoidlist = RelationGetIndexList(relation);
  
! 	/* allocate enough memory for all indexes */
! 	i = list_length(indexoidlist);
  
! 	if (i > 0)
! 		*Irel = (Relation *) palloc(i * sizeof(Relation));
  	else
  		*Irel = NULL;
  
+ 	/* collect just the ready indexes */
  	i = 0;
  	foreach(indexoidscan, indexoidlist)
  	{
  		Oid			indexoid = lfirst_oid(indexoidscan);
+ 		Relation	indrel;
  
! 		indrel = index_open(indexoid, lockmode);
! 		if (IndexIsReady(indrel->rd_index))
! 			(*Irel)[i++] = indrel;
! 		else
! 			index_close(indrel, lockmode);
  	}
  
+ 	*nindexes = i;
+ 
  	list_free(indexoidlist);
  }
  
diff --git a/src/backend/executor/execUtils.c b/src/backend/executor/execUtils.c
index 9f959c539e880ad90704b74205d2f1d91cd6d2f4..bfacc6c30acf6c16565a6b711590ec22969ecfb0 100644
*** a/src/backend/executor/execUtils.c
--- b/src/backend/executor/execUtils.c
*************** ExecOpenIndices(ResultRelInfo *resultRel
*** 907,912 ****
--- 907,915 ----
  	/*
  	 * For each index, open the index relation and save pg_index info. We
  	 * acquire RowExclusiveLock, signifying we will update the index.
+ 	 *
+ 	 * Note: we do this even if the index is not IndexIsReady; it's not worth
+ 	 * the trouble to optimize for the case where it isn't.
  	 */
  	i = 0;
  	foreach(l, indexoidlist)
diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index 77e701d2fc932d5bbfabccab3b552d038828fbac..d0a3f72351d236ba2b76b7e7c9cd47abd6a56ee5 100644
*** a/src/backend/optimizer/util/plancat.c
--- b/src/backend/optimizer/util/plancat.c
*************** get_relation_info(PlannerInfo *root, Oid
*** 166,174 ****
  			 * Ignore invalid indexes, since they can't safely be used for
  			 * queries.  Note that this is OK because the data structure we
  			 * are constructing is only used by the planner --- the executor
! 			 * still needs to insert into "invalid" indexes!
  			 */
! 			if (!index->indisvalid)
  			{
  				index_close(indexRelation, NoLock);
  				continue;
--- 166,175 ----
  			 * Ignore invalid indexes, since they can't safely be used for
  			 * queries.  Note that this is OK because the data structure we
  			 * are constructing is only used by the planner --- the executor
! 			 * still needs to insert into "invalid" indexes, if they're marked
! 			 * IndexIsReady.
  			 */
! 			if (!IndexIsValid(index))
  			{
  				index_close(indexRelation, NoLock);
  				continue;
diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index ae0ef91eb7fdb078f1187886fbb0523ae994e51c..9c39bacc64fb45979b678115942d456b8d2a6878 100644
*** a/src/backend/parser/parse_utilcmd.c
--- b/src/backend/parser/parse_utilcmd.c
*************** transformIndexConstraint(Constraint *con
*** 1525,1542 ****
  							index_name, RelationGetRelationName(heap_rel)),
  					 parser_errposition(cxt->pstate, constraint->location)));
  
! 		if (!index_form->indisvalid)
  			ereport(ERROR,
  					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
  					 errmsg("index \"%s\" is not valid", index_name),
  					 parser_errposition(cxt->pstate, constraint->location)));
  
- 		if (!index_form->indisready)
- 			ereport(ERROR,
- 					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
- 					 errmsg("index \"%s\" is not ready", index_name),
- 					 parser_errposition(cxt->pstate, constraint->location)));
- 
  		if (!index_form->indisunique)
  			ereport(ERROR,
  					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
--- 1525,1536 ----
  							index_name, RelationGetRelationName(heap_rel)),
  					 parser_errposition(cxt->pstate, constraint->location)));
  
! 		if (!IndexIsValid(index_form))
  			ereport(ERROR,
  					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
  					 errmsg("index \"%s\" is not valid", index_name),
  					 parser_errposition(cxt->pstate, constraint->location)));
  
  		if (!index_form->indisunique)
  			ereport(ERROR,
  					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 6026599a9bc5e089d4c89db3458a8994dc70d949..12a02b8c09b22bb21af897609129fdbb5d6e0433 100644
*** a/src/backend/utils/cache/relcache.c
--- b/src/backend/utils/cache/relcache.c
*************** RelationReloadIndexInfo(Relation relatio
*** 1731,1739 ****
--- 1731,1752 ----
  				 RelationGetRelid(relation));
  		index = (Form_pg_index) GETSTRUCT(tuple);
  
+ 		/*
+ 		 * Basically, let's just copy all the bool fields.  There are one or
+ 		 * two of these that can't actually change in the current code, but
+ 		 * it's not worth it to track exactly which ones they are.  None of
+ 		 * the array fields are allowed to change, though.
+ 		 */
+ 		relation->rd_index->indisunique = index->indisunique;
+ 		relation->rd_index->indisprimary = index->indisprimary;
+ 		relation->rd_index->indisexclusion = index->indisexclusion;
+ 		relation->rd_index->indimmediate = index->indimmediate;
+ 		relation->rd_index->indisclustered = index->indisclustered;
  		relation->rd_index->indisvalid = index->indisvalid;
  		relation->rd_index->indcheckxmin = index->indcheckxmin;
  		relation->rd_index->indisready = index->indisready;
+ 
+ 		/* Copy xmin too, as that is needed to make sense of indcheckxmin */
  		HeapTupleHeaderSetXmin(relation->rd_indextuple->t_data,
  							   HeapTupleHeaderGetXmin(tuple->t_data));
  
*************** RelationGetIndexList(Relation relation)
*** 3355,3361 ****
  		result = insert_ordered_oid(result, index->indexrelid);
  
  		/* Check to see if it is a unique, non-partial btree index on OID */
! 		if (index->indnatts == 1 &&
  			index->indisunique && index->indimmediate &&
  			index->indkey.values[0] == ObjectIdAttributeNumber &&
  			index->indclass.values[0] == OID_BTREE_OPS_OID &&
--- 3368,3375 ----
  		result = insert_ordered_oid(result, index->indexrelid);
  
  		/* Check to see if it is a unique, non-partial btree index on OID */
! 		if (IndexIsValid(index) &&
! 			index->indnatts == 1 &&
  			index->indisunique && index->indimmediate &&
  			index->indkey.values[0] == ObjectIdAttributeNumber &&
  			index->indclass.values[0] == OID_BTREE_OPS_OID &&
*************** RelationGetIndexAttrBitmap(Relation rela
*** 3662,3667 ****
--- 3676,3686 ----
  
  	/*
  	 * For each index, add referenced attributes to indexattrs.
+ 	 *
+ 	 * Note: we consider all indexes returned by RelationGetIndexList, even if
+ 	 * they are not indisready or indisvalid.  This is important because an
+ 	 * index for which CREATE INDEX CONCURRENTLY has just started must be
+ 	 * included in HOT-safety decisions (see README.HOT).
  	 */
  	indexattrs = NULL;
  	foreach(l, indexoidlist)
diff --git a/src/include/catalog/index.h b/src/include/catalog/index.h
index 4172f0e4006ddd989d13c993edef3be72b48dbc4..9c9987e8765408d3365642686ed4291ed61913be 100644
*** a/src/include/catalog/index.h
--- b/src/include/catalog/index.h
*************** typedef void (*IndexBuildCallback) (Rela
*** 27,32 ****
--- 27,39 ----
  												bool tupleIsAlive,
  												void *state);
  
+ /* Action code for index_set_state_flags */
+ typedef enum
+ {
+ 	INDEX_CREATE_SET_READY,
+ 	INDEX_CREATE_SET_VALID
+ } IndexStateFlagsAction;
+ 
  
  extern void index_check_primary_key(Relation heapRel,
  						IndexInfo *indexInfo,
*************** extern double IndexBuildHeapScan(Relatio
*** 88,93 ****
--- 95,102 ----
  
  extern void validate_index(Oid heapId, Oid indexId, Snapshot snapshot);
  
+ extern void index_set_state_flags(Oid indexId, IndexStateFlagsAction action);
+ 
  extern void reindex_index(Oid indexId, bool skip_constraint_checks);
  
  /* Flag bits for reindex_relation(): */
diff --git a/src/include/catalog/pg_index.h b/src/include/catalog/pg_index.h
index 9f446a5547ec7633fcea73c91c32acfcfe2552f0..dc372b82e26117bedf18dde58bfc95c9e3c9eeb9 100644
*** a/src/include/catalog/pg_index.h
--- b/src/include/catalog/pg_index.h
*************** typedef FormData_pg_index *Form_pg_index
*** 92,95 ****
--- 92,103 ----
  #define INDOPTION_DESC			0x0001	/* values are in reverse order */
  #define INDOPTION_NULLS_FIRST	0x0002	/* NULLs are first instead of last */
  
+ /*
+  * Use of these macros is recommended over direct examination of the state
+  * flag columns where possible; this allows source code compatibility with
+  * 9.2 and up.
+  */
+ #define IndexIsValid(indexForm) ((indexForm)->indisvalid)
+ #define IndexIsReady(indexForm) ((indexForm)->indisready)
+ 
  #endif   /* PG_INDEX_H */
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