Re: Implementing Incremental View Maintenance

Hi.

UNION query problem.(server crash)

When creating an INCREMENTAL MATERIALIZED VIEW,
the server process crashes if you specify a query with a UNION.

(commit id = 23151be7be8d8f8f9c35c2d0e4e5353aedf2b31e)

execute log.

```
[ec2-user(at)ip-10-0-1-10 ivm]$ psql testdb -e -f union_query_crash.sql
DROP TABLE IF EXISTS table_x CASCADE;
psql:union_query_crash.sql:6: NOTICE: drop cascades to view xy_union_v
DROP TABLE
DROP TABLE IF EXISTS table_y CASCADE;
DROP TABLE
CREATE TABLE table_x (id int, data numeric);
CREATE TABLE
CREATE TABLE table_y (id int, data numeric);
CREATE TABLE
INSERT INTO table_x VALUES (generate_series(1, 3), random()::numeric);
INSERT 0 3
INSERT INTO table_y VALUES (generate_series(1, 3), random()::numeric);
INSERT 0 3
SELECT * FROM table_x;
id | data
----+--------------------
1 | 0.950724735058774
2 | 0.0222670808201144
3 | 0.391258547114841
(3 rows)

SELECT * FROM table_y;
id | data
----+--------------------
1 | 0.991717347778337
2 | 0.0528458947672874
3 | 0.965044982911163
(3 rows)

CREATE VIEW xy_union_v AS
SELECT 'table_x' AS name, * FROM table_x
UNION
SELECT 'table_y' AS name, * FROM table_y
;
CREATE VIEW
TABLE xy_union_v;
name | id | data
---------+----+--------------------
table_y | 2 | 0.0528458947672874
table_x | 2 | 0.0222670808201144
table_y | 3 | 0.965044982911163
table_x | 1 | 0.950724735058774
table_x | 3 | 0.391258547114841
table_y | 1 | 0.991717347778337
(6 rows)

CREATE INCREMENTAL MATERIALIZED VIEW xy_imv AS
SELECT 'table_x' AS name, * FROM table_x
UNION
SELECT 'table_y' AS name, * FROM table_y
;
psql:union_query_crash.sql:28: server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
psql:union_query_crash.sql:28: fatal: connection to server was lost
```
UNION query problem.(server crash)

When creating an INCREMENTAL MATERIALIZED VIEW,
the server process crashes if you specify a query with a UNION.

(commit id = 23151be7be8d8f8f9c35c2d0e4e5353aedf2b31e)

execute log.

```
[ec2-user(at)ip-10-0-1-10 ivm]$ psql testdb -e -f union_query_crash.sql
DROP TABLE IF EXISTS table_x CASCADE;
psql:union_query_crash.sql:6: NOTICE: drop cascades to view xy_union_v
DROP TABLE
DROP TABLE IF EXISTS table_y CASCADE;
DROP TABLE
CREATE TABLE table_x (id int, data numeric);
CREATE TABLE
CREATE TABLE table_y (id int, data numeric);
CREATE TABLE
INSERT INTO table_x VALUES (generate_series(1, 3), random()::numeric);
INSERT 0 3
INSERT INTO table_y VALUES (generate_series(1, 3), random()::numeric);
INSERT 0 3
SELECT * FROM table_x;
id | data
----+--------------------
1 | 0.950724735058774
2 | 0.0222670808201144
3 | 0.391258547114841
(3 rows)

SELECT * FROM table_y;
id | data
----+--------------------
1 | 0.991717347778337
2 | 0.0528458947672874
3 | 0.965044982911163
(3 rows)

CREATE VIEW xy_union_v AS
SELECT 'table_x' AS name, * FROM table_x
UNION
SELECT 'table_y' AS name, * FROM table_y
;
CREATE VIEW
TABLE xy_union_v;
name | id | data
---------+----+--------------------
table_y | 2 | 0.0528458947672874
table_x | 2 | 0.0222670808201144
table_y | 3 | 0.965044982911163
table_x | 1 | 0.950724735058774
table_x | 3 | 0.391258547114841
table_y | 1 | 0.991717347778337
(6 rows)

CREATE INCREMENTAL MATERIALIZED VIEW xy_imv AS
SELECT 'table_x' AS name, * FROM table_x
UNION
SELECT 'table_y' AS name, * FROM table_y
;
psql:union_query_crash.sql:28: server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
psql:union_query_crash.sql:28: fatal: connection to server was lost
```

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>
>
>