Re: Lazy View's Column Computing

For all that we know, it may already be happening. That looks like a
pretty reasonable optimization which may already be in place. If we
create a view:

mgogala=# select * from dept;
 deptno |   dname    |   loc
--------+------------+----------
     10 | ACCOUNTING | NEW YORK
     20 | RESEARCH   | DALLAS
     30 | SALES      | CHICAGO
     40 | OPERATIONS | BOSTON
(4 rows)

mgogala=# create view acct_view as select * from emp where deptno=10;
CREATE VIEW

The query from the view would probably merge view  with the original and
optimize everything as a single query. Unfortunately, there is no way to
tell:

mgogala=# explain select ename,job,sal from acct_view;
                     QUERY PLAN
----------------------------------------------------
 Seq Scan on emp  (cost=0.00..1.18 rows=3 width=21)
   Filter: (deptno = 10)
(2 rows)

The only tool that you have at your disposal is EXPLAIN. What we need to
ascertain that assumption is an optimizer trace file detailing the
decisions made by optimizer, something like the event 10053 from another
database which will remain unnamed. Merging the view query into the top
level query would produce something like this:

mgogala=# select ename,job,sal from emp
mgogala-# where deptno=10;
 ename  |    job    | sal
--------+-----------+------
 CLARK  | MANAGER   | 2450
 KING   | PRESIDENT | 5000
 MILLER | CLERK     | 1300
(3 rows)

The table, shown below, has more columns than the 3 used in the above query:

mgogala=# \d emp
                           Table "mgogala.emp"
  Column  |            Type             | Collation | Nullable | Default
----------+-----------------------------+-----------+----------+---------
 empno    | smallint                    |           | not null |
 ename    | character varying(10)       |           |          |
 job      | character varying(9)        |           |          |
 mgr      | smallint                    |           |          |
 hiredate | timestamp without time zone |           |          |
 sal      | double precision            |           |          |
 comm     | double precision            |           |          |
 deptno   | smallint                    |           |          |
Indexes:
    "emp_pkey" PRIMARY KEY, btree (empno)
Foreign-key constraints:
    "fk_deptno" FOREIGN KEY (deptno) REFERENCES dept(deptno)

Merging the top level query with the view query would be smart tactic
which is probably already deployed. However, it is not possible to tell
with the tools at hand. That is what you want: the query touches only
the columns you need, nothing else. That is done by the query optimizer
in the "rewrite" phase of the query.

https://www.postgresql.org/docs/12/query-path.html

I could bet that the top level query gets merged with the view query
during the rewrite and that the columns that aren't needed aren't
touched. That in particular means that the function computing an
untouched column of the query isn't executed as it is.

Regards

Regards

On 8/2/21 10:12 AM, Avi Weinberg wrote:
>
> Hi,
>
> Is there a way to compute a column in a view only if it is referenced
> in the query?  I have a view's column that its value is computed by a
> function.  If in the query that column is not used at all, can
> Postgres "skip" computing it?
>
> Thanks!
>
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--
Mladen Gogala
Database Consultant
Tel: (347) 321-1217
https://dbwhisperer.wordpress.com