On Mon, 30 Aug 2010, Tom Lane wrote:
> Gerhard Wiesinger <lists@wiesinger.com> writes:
>> I know the drawbacks of an EAV design but I don't want to discuss that. I
>> want to discuss the major performance decrease of PostgreSQL 8.3
>> (performance was ok) to PostgreSQL 8.4 (performance is NOT ok).
>
>> Any further ideas how I can track this down?
>> Can someone explain the difference in query plan from an optimizer point
>> of view?
>
> Since you haven't shown us the 8.3 plan, it's kind of hard to speculate ;-)
>
> One thing that jumped out at me was that 8.4 appears to be expecting
> multiple matches in each of the left-joined tables, which is why the
> total rowcount estimate balloons so fast. I rather imagine that you are
> expecting at most one match in reality, else the query isn't going to
> behave nicely. Is this correct? Are you *sure* you analyzed all these
> tables? And if that is how the data looks, where is the actual
> performance problem? A bad rowcount estimate isn't in itself going
> to kill you.
>
> FWIW, in a similar albeit toy example, I don't see any difference
> between the 8.3 and 8.4 plans or cost estimates.
Yes, I'm expecting only one match in reality and I thing PostgreSQL should
also know that from table definition and constraints. Long answer below.
Query doesn't "end" in PostgreSQL.
From the definition:
CREATE TABLE value_types (
valuetypeid bigint PRIMARY KEY,
description varchar(256) NOT NULL -- e.g. 'float', 'integer', 'boolean'
);
CREATE TABLE key_description (
keyid bigint PRIMARY KEY,
description varchar(256) NOT NULL UNIQUE,
fk_valuetypeid bigint NOT NULL,
unit varchar(256) NOT NULL, -- e.g. '°C'
FOREIGN KEY(fk_valuetypeid) REFERENCES value_types(valuetypeid) ON DELETE RESTRICT
);
-- ALTER TABLE key_description DROP CONSTRAINT c_key_description_description;
-- ALTER TABLE key_description ADD CONSTRAINT c_key_description_description UNIQUE(description);
CREATE TABLE log (
id bigserial PRIMARY KEY,
datetime timestamp with time zone NOT NULL,
tdate date NOT NULL,
ttime time with time zone NOT NULL
);
CREATE TABLE log_details (
fk_id bigint NOT NULL,
fk_keyid bigint NOT NULL,
value double precision NOT NULL,
FOREIGN KEY (fk_id) REFERENCES log(id) ON DELETE CASCADE,
FOREIGN KEY (fk_keyid) REFERENCES key_description(keyid) ON DELETE RESTRICT,
CONSTRAINT unique_key_and_id UNIQUE(fk_id, fk_keyid)
);
Therefore keyid is unique and eg d1.fk_keyid is unique.
With constraint from log_details and d1.fk_keyid is unique fk_id is
unique for a given d1.fk_keyid.
BTW: I have the old data setup. /var/lib/pgsql-old. Is there a fast setup
with old version on different TCP port possible to compare query plans?
Thnx.
Ciao,
Gerhard
--
http://www.wiesinger.com/
