Good day,
I have a recursive CTE where a table scan occurs, even though there doesn't seem to be a good reason for it.
It seems the planner came to the conclusion that columns that are not actually used in the output, joins or a where clause are a part of the output.
It's not a performance problem now and the query runs quickly (which is why I haven't posted it on the performance mailing list). What bothers me is that the scan seems to be there without being really necessary and I would like to avoid any extra I/O. I would expect the plan to be close the simplified query without CTE posted after the CTE query below.
I'm also worried that this could get worse over time and that it might influence the query's performance.
The question:
Am I missing something and the table scan is necessary?
If not, is there a way to avoid it?
This is the query:
WITH RECURSIVE user_subordinates
AS
(
SELECT
ur1."id" AS var_id,
ur1.id_user_parent AS var_id_user_parent,
ur1.login AS var_login,
ur1_1.login AS var_login_parent,
1::smallint AS var_sort_order
FROM
"user" ur1
JOIN
"user" ur1_1
ON(ur1.id_user_parent=ur1_1."id")
WHERE
ur1.id = 3970
AND ur1.disabled=false
UNION ALL
SELECT
ur2."id" AS var_id,
ur2.id_user_parent AS var_id_user_parent,
ur2.login AS var_login,
ups1.var_login AS var_login_parent,
(ups1.var_sort_order + 1)::smallint AS var_sort_order
FROM
user_subordinates ups1
JOIN
"user" ur2
ON( ups1.var_id = ur2.id_user_parent
AND ups1.var_id <> ur2.id
)
)
SELECT
var_id,
var_id_user_parent,
var_login,
var_login_parent,
var_sort_order
FROM
user_subordinates
ORDER BY
var_sort_order,
var_id,
var_id_user_parent
;
This is its execution plan:
http://explain.depesz.com/s/4hY
This is a simplified version of the query without CTE:
SELECT
ur2."id" AS var_id,
ur2.id_user_parent AS var_id_user_parent,
ur2.login AS var_login,
ups1.var_login AS var_login_parent,
(ups1.var_sort_order + 1)::smallint AS var_sort_order
FROM
(
SELECT
ur1."id" AS var_id,
ur1.id_user_parent AS var_id_user_parent,
ur1.login AS var_login,
ur1_1.login AS var_login_parent,
1::smallint AS var_sort_order
FROM
"user" ur1
JOIN
"user" ur1_1
ON(ur1.id_user_parent=ur1_1."id")
WHERE
ur1.id = 3970
AND ur1.disabled=false
) ups1
JOIN
"user" ur2
ON( ups1.var_id = ur2.id_user_parent
AND ups1.var_id <> ur2.id
)
;
Its plan is here:
http://explain.depesz.com/s/Ak3
Here's the table's DDL:
CREATE TABLE "user"
(
id bigserial NOT NULL, -- Unique row identifier
id_user_parent bigint NOT NULL DEFAULT 3, -- Identifier of user's parent user
id_address bigint NOT NULL,
login character varying NOT NULL, -- Login name of a user
password character varying NOT NULL, -- Login password of a user
date_created timestamp without time zone NOT NULL DEFAULT now(), -- Date and time at which the record was created
id_user_created bigint NOT NULL, -- Identifier of a user who created the row
date_modified timestamp without time zone, -- Date and time at which the record was modified
id_user_modified bigint, -- Identifier of a user who modified the row
disabled boolean NOT NULL DEFAULT true, -- Its value is set to true when user account is disabled, false when enabled.
activation_hash character varying NOT NULL, -- Activation hash that a hashed string sent to the user matches against during activation.
invoiced boolean NOT NULL DEFAULT false, -- True is user wishes to be invoiced, false otherwise.
CONSTRAINT user_pkey PRIMARY KEY (id),
CONSTRAINT user_id_address_fkey FOREIGN KEY (id_address) REFERENCES address (id),
CONSTRAINT user_id_user_created_fkey FOREIGN KEY (id_user_created) REFERENCES "user" (id),
CONSTRAINT user_id_user_modified_fkey FOREIGN KEY (id_user_modified) REFERENCES "user" (id),
CONSTRAINT u_login UNIQUE (login)
);
CREATE INDEX fki_user_id_address_fkey ON "user" USING btree(id_address);
CREATE INDEX ix__user__id_user_parent ON "user" USING btree(id_user_parent);
CREATE UNIQUE INDEX ix__user__login ON "user" USING btree (lower(login::text) COLLATE pg_catalog."default");
CREATE UNIQUE INDEX ix__user__login__varchar_pattern_ops ON "user" USING btree (lower(login::text) COLLATE pg_catalog."default" varchar_pattern_ops);
Environment: Windows 7 Professional x64, PostgreSQL 9.3.1. The table has been analysed before executing the query and getting the explain result. It's on my workstation and nobody else uses the database but me.
Any thought on this are appreciated.
Thank you.
Peter Slapansky
