Re: (Re)building index using itself or another index of the same table - Mailing list pgsql-hackers

Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:

> On Thu, Sep 12, 2019 at 11:08:28AM -0400, Tom Lane wrote:

>>I have exactly no faith that this fixes things enough to make such
>>cases supportable.  And I have no interest in opening that can of
>>worms anyway.  I'd rather put in some code to reject database
>>accesses in immutable functions.
>>
>
> Same here. My hunch is a non-trivaial fraction of applications using
> this "trick" is silently broken in various subtle ways.

Ok, I see the point. However, "could not read block" error might seem
quite scary to the users; it looks like data corruption. How about
ERRORing out then in get_relation_info instead of skipping reindexing
indexes, like in attached? Even if this doesn't cover all cases, at
least one scenario observed in the field would have better error
message.

Rejecting database access completely in immutable functions would be
unfortunate for our particular case, because this GIN index on
expression joining the very indexed table multiple times (and using thus
btree index) is, well, useful. Here is a brief description of the
case. Indexed table stores postal addresses, which are of hierarchical
nature (e.g. country-region-city-street-house). Single row is one element
of any depth (e.g. region or house); each row stores link to its parent
in parent_guid column, establishing thus the hierarchy
(e.g. house has link to the street).

The task it to get the full address by typing random parts of it
(imagine typing hints in Google Maps). For that, FTS is used. GIN index
is built on full addresses, and to get the full address table is climbed
up about six times (hierarchy depth) by following parent_guid chain.

We could materialize full addresses in the table and eliminate the need
to form them in the index expression, but that would seriously increase
amount of required storage -- GIN doesn't store indexed columns fully,
and thus it is cheaper to 'materialize' full addresses inside it only.


Surely this is a hack which cheats the system. We might imagine creating
some functionality (kinda index referring to multiple rows of the table
-- or even rows of different tables) making it unneccessary, but such
functionality doesn't exist today, and the hack is useful, if you
understand the risk.


>>> One might argue that function selecting from table can hardly be called
>>> immutable, and immutability is required for index expressions. However,
>>> if user is sure table contents doesn't change, why not?
>>
>>If the table contents never change, why are you doing VACUUM FULL on it?
>>
>
> It's possible the columns referenced by the index expression are not
> changing, but some additional columns are updated.

Yeah. Also table can be CLUSTERed without VACUUM FULL.


--
Arseny Sher
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

From f5b9c433bf387a9ddbe318dfea2b96c02c4a945e Mon Sep 17 00:00:00 2001
From: Arseny Sher <sher-ars@yandex.ru>
Date: Thu, 12 Sep 2019 17:35:16 +0300
Subject: [PATCH] ERROR out early on attempt to touch user indexes while they
 are being (re)built.

Existing ReindexIsProcessingIndex check is consulted only in genam.c and thus
enforced only for system catalogs. Check it also in the planner, so that indexes
which are currently being rebuilt are never tried. Also cock SetReindexProcessing
in index_create to defend from index self usage during its creation.

Without this, VACUUM FULL or just CREATE INDEX might fail with something like

ERROR:  could not read block 3534 in file "base/41366676/56697497": read only 0 of 8192 bytes

if there are indexes which usage can be considered during these very
indexes (re)building, i.e. index expression scans indexed table.

Such error might seem scary, so catch this earlier.
---
 src/backend/catalog/index.c                | 22 ++++++++++++++++++++--
 src/backend/optimizer/util/plancat.c       |  9 +++++++++
 src/test/regress/expected/create_index.out | 15 +++++++++++++++
 src/test/regress/sql/create_index.sql      | 13 +++++++++++++
 4 files changed, 57 insertions(+), 2 deletions(-)

diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index 3e1d40662d..5bc764ce46 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -1174,7 +1174,22 @@ index_create(Relation heapRelation,
     }
     else
     {
-        index_build(heapRelation, indexRelation, indexInfo, false, true);
+        /* ensure SetReindexProcessing state isn't leaked */
+        PG_TRY();
+        {
+            /* Suppress use of the target index while building it */
+            SetReindexProcessing(heapRelationId, indexRelationId);
+
+            index_build(heapRelation, indexRelation, indexInfo, false, true);
+        }
+        PG_CATCH();
+        {
+            /* Make sure flag gets cleared on error exit */
+            ResetReindexProcessing();
+            PG_RE_THROW();
+        }
+        PG_END_TRY();
+        ResetReindexProcessing();
     }
 
     /*
@@ -1379,7 +1394,10 @@ index_concurrently_build(Oid heapRelationId,
     indexInfo->ii_Concurrent = true;
     indexInfo->ii_BrokenHotChain = false;
 
-    /* Now build the index */
+    /*
+     * Now build the index
+     * SetReindexProcessing is not required since indisvalid is false anyway
+     */
     index_build(heapRel, indexRelation, indexInfo, false, true);
 
     /* Close both the relations, but keep the locks */
diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index cf1761401d..e42471cac8 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -27,6 +27,7 @@
 #include "access/xlog.h"
 #include "catalog/catalog.h"
 #include "catalog/dependency.h"
+#include "catalog/index.h"
 #include "catalog/heap.h"
 #include "catalog/pg_am.h"
 #include "catalog/pg_proc.h"
@@ -199,6 +200,14 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
             indexRelation = index_open(indexoid, lmode);
             index = indexRelation->rd_index;
 
+            if (ReindexIsProcessingIndex(indexoid))
+                ereport(ERROR,
+                        (errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+                         errmsg("cannot use index %s which is currently being reindexed",
+                                RelationGetRelationName(indexRelation)),
+                         errhint("Probably index expressions include functions "
+                                 "accessing indexed table itself, but they must be immutable.")));
+
             /*
              * Ignore invalid indexes, since they can't safely be used for
              * queries.  Note that this is OK because the data structure we
diff --git a/src/test/regress/expected/create_index.out b/src/test/regress/expected/create_index.out
index 324db1b6ae..f9aeb97ed8 100644
--- a/src/test/regress/expected/create_index.out
+++ b/src/test/regress/expected/create_index.out
@@ -1323,6 +1323,21 @@ create unique index hash_f8_index_1 on hash_f8_heap(abs(random));
 create unique index hash_f8_index_2 on hash_f8_heap((seqno + 1), random);
 create unique index hash_f8_index_3 on hash_f8_heap(random) where seqno > 1000;
 --
+-- Create an index which might consider using this very index during the build.
+--
+-- primary key ensures relhasindex is set
+CREATE TABLE pears(f1 int primary key, f2 int);
+INSERT INTO pears SELECT i, i+1 FROM generate_series(1, 100) i;
+CREATE FUNCTION pears_f(i int) RETURNS int LANGUAGE SQL IMMUTABLE AS $$
+  SELECT f1 FROM pears WHERE pears.f2 = 42
+$$;
+CREATE index ON pears ((pears_f(f1)));
+ERROR:  cannot use index pears_pears_f_idx which is currently being reindexed
+HINT:  Probably index expressions include functions accessing indexed table itself, but they must be immutable.
+CONTEXT:  SQL function "pears_f" during startup
+DROP TABLE pears;
+DROP FUNCTION pears_f;
+--
 -- Try some concurrent index builds
 --
 -- Unfortunately this only tests about half the code paths because there are
diff --git a/src/test/regress/sql/create_index.sql b/src/test/regress/sql/create_index.sql
index f96bebf410..76a781f6b0 100644
--- a/src/test/regress/sql/create_index.sql
+++ b/src/test/regress/sql/create_index.sql
@@ -446,6 +446,19 @@ create unique index hash_f8_index_2 on hash_f8_heap((seqno + 1), random);
 create unique index hash_f8_index_3 on hash_f8_heap(random) where seqno > 1000;
 
 --
+-- Create an index which might consider using this very index during the build.
+--
+-- primary key ensures relhasindex is set
+CREATE TABLE pears(f1 int primary key, f2 int);
+INSERT INTO pears SELECT i, i+1 FROM generate_series(1, 100) i;
+CREATE FUNCTION pears_f(i int) RETURNS int LANGUAGE SQL IMMUTABLE AS $$
+  SELECT f1 FROM pears WHERE pears.f2 = 42
+$$;
+CREATE index ON pears ((pears_f(f1)));
+DROP TABLE pears;
+DROP FUNCTION pears_f;
+
+--
 -- Try some concurrent index builds
 --
 -- Unfortunately this only tests about half the code paths because there are
-- 
2.11.0