Re: Implementing Incremental View Maintenance

On Sat, 11 Jan 2020 09:27:58 +0900
nuko yokohama <nuko(dot)yokohama(at)gmail(dot)com> wrote:

> LIMIT clause without ORDER BY should be prohibited when creating
> incremental materialized views.
>
> In SQL, the result of a LIMIT clause without ORDER BY is undefined.
> If the LIMIT clause is allowed when creating an incremental materialized
> view, incorrect results will be obtained when the view is updated after
> updating the source table.

Thank you for your advice. It's just as you said.
LIMIT/OFFSET clause should is prohibited. We will add this to next patch.

Best Regards,
Takuma Hoshiai

>
> ```
> [ec2-user(at)ip-10-0-1-10 ivm]$ psql --version
> psql (PostgreSQL) 13devel-ivm-3bf6953688153fa72dd48478a77e37cf3111a1ee
> [ec2-user(at)ip-10-0-1-10 ivm]$ psql testdb -e -f limit-problem.sql
> DROP TABLE IF EXISTS test CASCADE;
> psql:limit-problem.sql:1: NOTICE: drop cascades to materialized view
> test_imv
> DROP TABLE
> CREATE TABLE test (id int primary key, data text);
> CREATE TABLE
> INSERT INTO test VALUES (generate_series(1, 10), 'foo');
> INSERT 0 10
> CREATE INCREMENTAL MATERIALIZED VIEW test_imv AS SELECT * FROM test LIMIT 1;
> SELECT 1
> Materialized view "public.test_imv"
> Column | Type | Collation | Nullable | Default | Storage |
> Stats target | Description
> ---------------+---------+-----------+----------+---------+----------+--------------+-------------
> id | integer | | | | plain |
> |
> data | text | | | | extended |
> |
> __ivm_count__ | bigint | | | | plain |
> |
> View definition:
> SELECT test.id,
> test.data
> FROM test
> LIMIT 1;
> Access method: heap
> Incremental view maintenance: yes
>
> SELECT * FROM test LIMIT 1;
> id | data
> ----+------
> 1 | foo
> (1 row)
>
> TABLE test_imv;
> id | data
> ----+------
> 1 | foo
> (1 row)
>
> UPDATE test SET data = 'bar' WHERE id = 1;
> UPDATE 1
> SELECT * FROM test LIMIT 1;
> id | data
> ----+------
> 2 | foo
> (1 row)
>
> TABLE test_imv;
> id | data
> ----+------
> 1 | bar
> (1 row)
>
> DELETE FROM test WHERE id = 1;
> DELETE 1
> SELECT * FROM test LIMIT 1;
> id | data
> ----+------
> 2 | foo
> (1 row)
>
> TABLE test_imv;
> id | data
> ----+------
> (0 rows)
> ```
>
> ORDER BY clause is not allowed when executing CREATE INCREMENTAL
> MATELIARIZED VIEW.
> We propose not to allow LIMIT clauses as well.
>
>
> 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>
> >
> >

--
Takuma Hoshiai <hoshiai(at)sraoss(dot)co(dot)jp>