On Thu, Aug 14, 2025 at 12:04:12PM +1000, Peter Smith wrote:
> On Fri, Aug 1, 2025 at 5:43 PM Japin Li <japi...@hotmail.com> wrote:
> >
> > On Fri, Aug 01, 2025 at 05:18:11PM +1000, Peter Smith wrote:
> > > On Wed, Jul 30, 2025 at 9:07 PM Japin Li <japi...@hotmail.com> wrote:
> > > >
> > > ...
> > > > 3.
> > > > I've also found that the VCI index is not working. Is this the expected
> > > > behavior?
> > > >
> > > > [local]:3209161 postgres=# \d+ t
> > > > Table "public.t"
> > > > Column | Type | Collation | Nullable | Default | Storage |
> > > > Compression | Stats target | Description
> > > > --------+---------+-----------+----------+---------+----------+-------------+--------------+-------------
> > > > id | integer | | | | plain |
> > > > | |
> > > > info | text | | | | extended |
> > > > | |
> > > > Indexes:
> > > > "t_id_idx" vci (id)
> > > > Access method: heap
> > > >
> > > > [local]:3209161 postgres=# SET enable_seqscan TO off;
> > > > SET
> > > > [local]:3209161 postgres=# EXPLAIN SELECT * FROM t WHERE id = 1000;
> > > > QUERY PLAN
> > > > -----------------------------------------------------
> > > > Seq Scan on t (cost=0.00..2084.00 rows=1 width=37)
> > > > Disabled: true
> > > > Filter: (id = 1000)
> > > > (3 rows)
> > > >
> > >
> > > Hi Japin. Yes, that's expected behaviour.
> > >
> > > VCI is used only when the vci index is defined for all the columns of
> > > your query. In your example there was a table with 2 columns ('id' and
> > > 'info') but you only have an index on the 'id' column. If you change
> > > the query then you can see VCI getting used.
> > >
> > > E.g.
> > >
> > > postgres=# EXPLAIN SELECT id FROM t WHERE id = 1000;
> > > QUERY PLAN
> > > ----------------------------------------------------------------------------
> > > Custom Scan (VCI Scan) using tidx on t (cost=0.00..209.00 rows=1
> > > width=4)
> > > Filter: (id = 1000)
> > > (2 rows)
> > >
> > > postgres=# EXPLAIN SELECT * FROM t WHERE id = 1000;
> > > QUERY PLAN
> > > ----------------------------------------------------
> > > Seq Scan on t (cost=0.00..209.00 rows=1 width=37)
> > > Filter: (id = 1000)
> > > (2 rows)
> > >
> > > postgres=# EXPLAIN SELECT id,info FROM t WHERE id = 1000;
> > > QUERY PLAN
> > > ----------------------------------------------------
> > > Seq Scan on t (cost=0.00..209.00 rows=1 width=37)
> > > Filter: (id = 1000)
> > > (2 rows)
> > >
> > > ~~~
> > >
> > > You can see this also in the DEBUG logs, from
> > > vci_can_rewrite_custom_scan(), where it checks to see if the attrs are
> > > in the vci index or not.
> > >
> > > e.g.
> > > 2025-08-01 16:58:20.939 AEST [26528] DEBUG: vci index: target table
> > > "t"(oid=16384) tuples(rows=10000,extents=0)
> > > 2025-08-01 16:58:20.939 AEST [26528] DEBUG: vci index: don't match
> > > index "tidx"(oid=16469)
> > > 2025-08-01 16:58:20.940 AEST [26528] DEBUG: attrnum = 1 x
> > > 2025-08-01 16:58:20.940 AEST [26528] DEBUG: attrnum = 2
> > >
> >
> > Thanks for your explantion! Got it.
> >
> > Are there any plans to remove this restriction in the future?
> >
>
> No. There aren't any plans to remove this restriction because it is
> not considered to be a "restriction" in the first place; e.g. VCI is
> intended more like an accelerator only for those *specified* columns
> for which you intend to do your analysis.
>
Thanks for your explantion!
--
Best regards,
Japin Li
ChengDu WenWu Information Technology Co., LTD.
