Date: Mon, 6 Apr 2020 23:59:29 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Rick Vincent <rvincent@temenos.com>
Cc: pgsql-performance@postgresql.org, Manoj Kumar <manojkumar@temenos.com>,
	Herve Aubert <haubert@temenos.com>
Subject: Re: Postgres not using index on views
Message-ID: <20200407045929.GJ2228@telsasoft.com>
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On Mon, Apr 06, 2020 at 02:19:59PM +0000, Rick Vincent wrote:
> I am seeing a performance problem with postgresql v 11.7 on views, and I am wondering if anyone can tell me why or has any suggestion.
> 
> A table is created as:
> 
> CREATE TABLE "FBNK_CUSTOMER" (RECID VARCHAR(255) NOT NULL PRIMARY KEY, XMLRECORD VARCHAR)
> 
> And contains only 180 rows.
> 
> Doing an explain plan on the view created over this gives:
> 
> EXPLAIN ANALYZE
> select RECID from "V_FBNK_CUSTOMER"
> 
> 
> Subquery Scan on "V_FBNK_CUSTOMER"  (cost=0.00..19014.60 rows=180 width=7) (actual time=459.601..78642.189 rows=180 loops=1)
>   ->  Seq Scan on "FBNK_CUSTOMER" a  (cost=0.00..19012.80 rows=180 width=14575) (actual time=459.600..78641.950 rows=180 loops=1)
> 
> Yet an Explain plan on the underlying table( on select RECID from "FBNK_CUSTOMER") gives:
> 
> Seq Scan on "FBNK_CUSTOMER"  (cost=0.00..22.80 rows=180 width=7) (actual time=0.004..0.272 rows=180 loops=1)

It still did a seq scan on the table, so I'm not sure what this has to do with
index scans ?

> The following query takes an extremely long time for only 180 rows, and what this means is that we would have to index anything appearing in the where clause for every table in order to use views because the views seem not to consider the select clause.  Why is that and does anyone know a way around this?

Is there a reason why you don't store the extracted value in its own column ?
And maybe keep it up to date using an insert/update trigger on the xmlrecord
column.

-- 
Justin