Re: Implementing Incremental View Maintenance

On Tue, 4 Feb 2020 18:40:45 +0900
nuko yokohama <nuko(dot)yokohama(at)gmail(dot)com> wrote:

> "ROW LEVEL SECURITY" and INCREMENTAL MATERIALIZED VIEW.
>
> Hi.
>
> If ROW LEVEL SECURITY is set for the source table after creating the
> INCREMENTAL MATELIALIZED VIEW, the search results by that are not reflected.
> After setting ROW LEVEL SECURITY (similar to normal MATERIALIZED VIEW), you
> need to execute REFRESH MATERILALIZED VIEW and reflect the result.
> (Not limited to this, but in general cases where search results change by
> means other than DML)
>
> I propose to add this note to the document (rules.sgml).

Thank you for your suggestion! We'll add some description
about this to the documentation.

Regards,
Yugo Nagata

>
> execute log.
>
> ```
> [ec2-user(at)ip-10-0-1-10 rls]$ psql testdb -e -f rls.sql
> CREATE USER user_a;
> CREATE ROLE
> CREATE TABLE test (id int, data text);
> CREATE TABLE
> GRANT ALL ON TABLE test TO user_a;
> GRANT
> GRANT ALL ON SCHEMA public TO user_a;
> GRANT
> SET ROLE user_a;
> SET
> INSERT INTO test VALUES (1,'A'),(2,'B'),(3,'C');
> INSERT 0 3
> SELECT * FROM test;
> id | data
> ----+------
> 1 | A
> 2 | B
> 3 | C
> (3 rows)
>
> CREATE VIEW test_v AS SELECT * FROM test;
> CREATE VIEW
> CREATE MATERIALIZED VIEW test_mv AS SELECT * FROM test;
> SELECT 3
> CREATE INCREMENTAL MATERIALIZED VIEW test_imv AS SELECT * FROM test;
> SELECT 3
> SELECT * FROM test_v;
> id | data
> ----+------
> 1 | A
> 2 | B
> 3 | C
> (3 rows)
>
> SELECT * FROM test_mv;
> id | data
> ----+------
> 1 | A
> 2 | B
> 3 | C
> (3 rows)
>
> SELECT * FROM test_imv;
> id | data
> ----+------
> 3 | C
> 1 | A
> 2 | B
> (3 rows)
>
> RESET ROLE;
> RESET
> CREATE POLICY test_AAA ON test FOR SELECT TO user_a USING (data = 'A');
> CREATE POLICY
> ALTER TABLE test ENABLE ROW LEVEL SECURITY ;
> ALTER TABLE
> SET ROLE user_a;
> SET
> SELECT * FROM test_v;
> id | data
> ----+------
> 1 | A
> (1 row)
>
> SELECT * FROM test_mv;
> id | data
> ----+------
> 1 | A
> 2 | B
> 3 | C
> (3 rows)
>
> SELECT * FROM test_imv;
> id | data
> ----+------
> 3 | C
> 1 | A
> 2 | B
> (3 rows)
>
> REFRESH MATERIALIZED VIEW test_mv;
> REFRESH MATERIALIZED VIEW
> REFRESH MATERIALIZED VIEW test_imv;
> REFRESH MATERIALIZED VIEW
> SELECT * FROM test_mv;
> id | data
> ----+------
> 1 | A
> (1 row)
>
> SELECT * FROM test_imv;
> id | data
> ----+------
> 1 | A
> (1 row)
>
> RESET ROLE;
> RESET
> REVOKE ALL ON TABLE test FROM user_a;
> REVOKE
> REVOKE ALL ON TABLE test_v FROM user_a;
> REVOKE
> REVOKE ALL ON TABLE test_mv FROM user_a;
> REVOKE
> REVOKE ALL ON TABLE test_imv FROM user_a;
> REVOKE
> REVOKE ALL ON SCHEMA public FROM user_a;
> REVOKE
> DROP TABLE test CASCADE;
> psql:rls.sql:40: NOTICE: drop cascades to 3 other objects
> DETAIL: drop cascades to view test_v
> drop cascades to materialized view test_mv
> drop cascades to materialized view test_imv
> DROP TABLE
> DROP USER user_a;
> DROP ROLE
> [ec2-user(at)ip-10-0-1-10 rls]$
>
> ```
>
> Regard.
>
> 2018年12月27日(木) 21:57 Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>:
>
> > Hi,
> >
> > I would like to implement Incremental View Maintenance (IVM) on
> > PostgreSQL.
> > IVM is a technique to maintain materialized views which computes and
> > applies
> > only the incremental changes to the materialized views rather than
> > recomputate the contents as the current REFRESH command does.
> >
> > I had a presentation on our PoC implementation of IVM at PGConf.eu 2018
> > [1].
> > Our implementation uses row OIDs to compute deltas for materialized
> > views.
> > The basic idea is that if we have information about which rows in base
> > tables
> > are contributing to generate a certain row in a matview then we can
> > identify
> > the affected rows when a base table is updated. This is based on an idea of
> > Dr. Masunaga [2] who is a member of our group and inspired from ID-based
> > approach[3].
> >
> > In our implementation, the mapping of the row OIDs of the materialized view
> > and the base tables are stored in "OID map". When a base relation is
> > modified,
> > AFTER trigger is executed and the delta is recorded in delta tables using
> > the transition table feature. The accual udpate of the matview is triggerd
> > by REFRESH command with INCREMENTALLY option.
> >
> > However, we realize problems of our implementation. First, WITH OIDS will
> > be removed since PG12, so OIDs are no longer available. Besides this, it
> > would
> > be hard to implement this since it needs many changes of executor nodes to
> > collect base tables's OIDs during execuing a query. Also, the cost of
> > maintaining
> > OID map would be high.
> >
> > For these reasons, we started to think to implement IVM without relying on
> > OIDs
> > and made a bit more surveys.
> >
> > We also looked at Kevin Grittner's discussion [4] on incremental matview
> > maintenance. In this discussion, Kevin proposed to use counting algorithm
> > [5]
> > to handle projection views (using DISTNICT) properly. This algorithm need
> > an
> > additional system column, count_t, in materialized views and delta tables
> > of
> > base tables.
> >
> > However, the discussion about IVM is now stoped, so we would like to
> > restart and
> > progress this.
> >
> >
> > Through our PoC inplementation and surveys, I think we need to think at
> > least
> > the followings for implementing IVM.
> >
> > 1. How to extract changes on base tables
> >
> > I think there would be at least two approaches for it.
> >
> > - Using transition table in AFTER triggers
> > - Extracting changes from WAL using logical decoding
> >
> > In our PoC implementation, we used AFTER trigger and transition tables,
> > but using
> > logical decoding might be better from the point of performance of base
> > table
> > modification.
> >
> > If we can represent a change of UPDATE on a base table as query-like
> > rather than
> > OLD and NEW, it may be possible to update the materialized view directly
> > instead
> > of performing delete & insert.
> >
> >
> > 2. How to compute the delta to be applied to materialized views
> >
> > Essentially, IVM is based on relational algebra. Theorically, changes on
> > base
> > tables are represented as deltas on this, like "R <- R + dR", and the
> > delta on
> > the materialized view is computed using base table deltas based on "change
> > propagation equations". For implementation, we have to derive the
> > equation from
> > the view definition query (Query tree, or Plan tree?) and describe this as
> > SQL
> > query to compulte delta to be applied to the materialized view.
> >
> > There could be several operations for view definition: selection,
> > projection,
> > join, aggregation, union, difference, intersection, etc. If we can
> > prepare a
> > module for each operation, it makes IVM extensable, so we can start a
> > simple
> > view definition, and then support more complex views.
> >
> >
> > 3. How to identify rows to be modifed in materialized views
> >
> > When applying the delta to the materialized view, we have to identify
> > which row
> > in the matview is corresponding to a row in the delta. A naive method is
> > matching
> > by using all columns in a tuple, but clearly this is unefficient. If
> > thematerialized
> > view has unique index, we can use this. Maybe, we have to force
> > materialized views
> > to have all primary key colums in their base tables. In our PoC
> > implementation, we
> > used OID to identify rows, but this will be no longer available as said
> > above.
> >
> >
> > 4. When to maintain materialized views
> >
> > There are two candidates of the timing of maintenance, immediate (eager)
> > or deferred.
> >
> > In eager maintenance, the materialized view is updated in the same
> > transaction
> > where the base table is updated. In deferred maintenance, this is done
> > after the
> > transaction is commited, for example, when view is accessed, as a response
> > to user
> > request, etc.
> >
> > In the previous discussion[4], it is planned to start from "eager"
> > approach. In our PoC
> > implementaion, we used the other aproach, that is, using REFRESH command
> > to perform IVM.
> > I am not sure which is better as a start point, but I begin to think that
> > the eager
> > approach may be more simple since we don't have to maintain base table
> > changes in other
> > past transactions.
> >
> > In the eager maintenance approache, we have to consider a race condition
> > where two
> > different transactions change base tables simultaneously as discussed in
> > [4].
> >
> >
> > [1]
> > https://www.postgresql.eu/events/pgconfeu2018/schedule/session/2195-implementing-incremental-view-maintenance-on-postgresql/
> > [2]
> > https://ipsj.ixsq.nii.ac.jp/ej/index.php?active_action=repository_view_main_item_detail&page_id=13&block_id=8&item_id=191254&item_no=1
> > (Japanese only)
> > [3] https://dl.acm.org/citation.cfm?id=2750546
> > [4]
> > https://www.postgresql.org/message-id/flat/1368561126.64093.YahooMailNeo%40web162904.mail.bf1.yahoo.com
> > [5] https://dl.acm.org/citation.cfm?id=170066
> >
> > Regards,
> > --
> > Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>
> >
> >

--
Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp>