Re: Redundant sub query triggers slow nested loop left join - Mailing list pgsql-performance

>Well, that's darn odd.  It should not be getting that so far wrong.
>What's the datatype of the status column exactly (I'm guessing varchar
>but maybe not)?  Would you show us the pg_stats row for the status column?

It has been created as a char(1) in fact. The pg_stats row for the status
column is:

public|banners_links|status|0|5|2|{0,1}|{0.626397,0.373603}||0.560611

>I'm not sure why, but I think it must have something to do
>with the subquery structure of your query.  Were you showing us the
>whole truth about your query, or were there details you left out?

The query I gave in the opening post was just a small part, the part that I
initially identified as the 'slow path'. The last plan I gave was from the
whole query, without any details left out. I didn't gave the SQL of that
yet, so here it is:

SELECT
    id,
    status,
    merchant_id,
    description,
    org_text,
    users_banners_id,
    banner_url,
    cookie_redirect,
    type,

    CASE WHEN special_deal IS null THEN
        ''
    ELSE
        'special deal'
    END AS special_deal,

    CASE WHEN url_of_banner IS null    THEN
        ''
    ELSE
        url_of_banner
    END AS url_of_banner,

    CASE WHEN period_end IS NULL THEN
        'not_active'
    ELSE
        'active'
    END AS active_not_active,

    CASE WHEN ecpc IS NULL THEN
        0.00
    ELSE
        ROUND(ecpc::numeric,2)
    END AS ecpc,

    CASE WHEN ecpc_merchant IS NULL THEN
        0.00
    ELSE
        ROUND(ecpc_merchant::numeric,2)
    END AS ecpc_merchant

FROM
    /* SUBQUERY grand_total_fetch_banners */ (
        /* SUBQUERY grand_total  */(
            /* SUBQUERY banners_special_deals */    (

                /* SUBQUERY banners */ (
                    SELECT
                        *
                    FROM
                        /* SUBQUERY banners_links */ (
                            SELECT
                                banners_links.id,
                                merchant_id,
                                banners_org.banner_text AS org_text,
                                description,
                                status,
                                banner_url,
                                ecpc,
                                ecpc_merchant,
                                COALESCE(cookie_redirect,0) AS cookie_redirect
                            FROM
                                /* SUBQUERY banners_links  */ (

                                    /* subselect tot join ecpc_per_banner_links on banners_links*/
                                    /* SUBQUERY banners_links */ (
                                        SELECT
                                            *
                                        FROM
                                            banners_links
                                        WHERE
                                            merchant_id = 5631
                                    ) AS banners_links

                                        LEFT OUTER JOIN

                                    /* SUBQUERY ecpc_per_banner_link */    (
                                        SELECT
                                            CASE WHEN clicks_total > 0 THEN
                                                (revenue_total_affiliate/clicks_total)::float/1000.0
                                            ELSE
                                                0.0
                                            END AS ecpc,
                                            CASE WHEN clicks_total > 0 THEN
                                                (revenue_total/clicks_total)::float/1000.0
                                            ELSE
                                                0.0
                                            END AS ecpc_merchant,
                                            banners_links_id
                                        FROM
                                            precalculated_stats_banners_links
                                        WHERE
                                            status = 4            AND
                                            banners_links_id IN /* SUBQUERY */ (
                                                SELECT
                                                    id
                                                FROM
                                                    banners_links
                                                WHERE
                                                    merchant_id = 5631
                                            )
                                        ORDER BY
                                            ecpc DESC
                                    ) AS ecpc_per_banner_link

                                        ON (banners_links.id = ecpc_per_banner_link.banners_links_id)
                                ) AS banners_links

                                    ,

                                banners_org

                            WHERE
                                merchant_id = 5631                            AND
                                banners_links.id = banners_org.id_banner            AND
                                (banners_links.id = -1 OR -1 = -1)    AND
                                (banners_links.status = 0 OR 0 = -1)
                        ) AS banners_links

                            LEFT OUTER JOIN

                        /* SUBQUERY users_banners_tot_sub */(
                            SELECT
                                MAX (users_banners_id) AS users_banners_id,
                                merchant_users_banners_id,
                                banner_id
                            FROM
                                /* SUBQUERY users_banners_rotations_sub */(
                                    SELECT
                                        affiliate_id         AS merchant_users_banners_id,
                                        users_banners.id     AS users_banners_id,
                                        users_banners_rotation.banner_id
                                    FROM
                                        users_banners, users_banners_rotation
                                    WHERE
                                        affiliate_id = 5631                                    AND
                                        users_banners_rotation.users_banners_id = users_banners.id    AND
                                        users_banners.status = 3
                                ) AS users_banners_rotations_sub
                            GROUP BY
                                merchant_users_banners_id,banner_id
                        ) AS users_banners_tot_sub

                            ON (
                                banners_links.id = users_banners_tot_sub.banner_id     AND
                                banners_links.merchant_id =
users_banners_tot_sub.merchant_users_banners_id
                            )
                    ) AS banners

                        LEFT OUTER JOIN

                    /* SUBQUERY special_deals */(
                        SELECT
                            banner_deals.banner_id     AS id,
                            MAX(affiliate_id)         AS special_deal
                        FROM
                            banner_deals
                        GROUP BY
                            banner_deals.banner_id
                    ) AS special_deals

                        USING (id)

            ) AS banners_special_deals

                LEFT OUTER JOIN

            /* SUBQUERY types */ (
                SELECT
                    banner_types.id                 AS type_id,
                    banner_types.type                 AS type,
                    banners_banner_types.banner_id     AS id
                FROM
                    banner_types,banners_banner_types
                WHERE
                    banners_banner_types.banner_id IN /* SUBQUERY */ (
                        SELECT
                            id
                        FROM
                            banners_links
                        WHERE
                            merchant_id = 5631
                    ) AND
                    banners_banner_types.type_id = banner_types.id
            ) AS types

                USING (id)

        )  as grand_total

            LEFT OUTER JOIN

        /* SUBQUERY fetch_banners */ (
            SELECT
                banners_links_id AS id,
                url_of_banner
            FROM
                fetch_banners
        ) AS fetch_banners

            USING (id)
    ) AS grand_total_fetch_banners

        LEFT OUTER JOIN

    /* SUBQUERY active_banners */ (
        SELECT
            banner_id AS id,
            period_end
        FROM
            reward_ratings
        WHERE
            now() BETWEEN period_start AND period_end
        AND
            banner_id IN /* SUBQUERY */ (
                SELECT
                    id
                FROM
                    banners_links
                WHERE
                    merchant_id = 5631
            )
    ) AS active_banners

        USING (id)

WHERE
    (type_id =  -1 OR -1 = -1 )    AND
    (special_deal IS null)

ORDER BY
    id DESC

This is the original query without even the earlier mentioned redundant
check removed. For this query, PG 8.2 creates the following plan:

Sort  (cost=5094.40..5094.41 rows=1 width=597) (actual
time=15282.503..15282.734 rows=553 loops=1)
  Sort Key: public.banners_links.id
  ->  Nested Loop Left Join  (cost=3883.68..5094.39 rows=1 width=597)
(actual time=64.066..15280.773 rows=553 loops=1)
        Join Filter: (public.banners_links.id = reward_ratings.banner_id)
        ->  Nested Loop Left Join  (cost=2926.37..3486.98 rows=1 width=589)
(actual time=51.992..9231.245 rows=553 loops=1)
              Join Filter: (public.banners_links.id =
public.fetch_banners.banners_links_id)
              ->  Nested Loop Left Join  (cost=2926.37..3483.00 rows=1
width=519) (actual time=51.898..9183.007 rows=553 loops=1)
                    Join Filter: (public.banners_links.id =
ecpc_per_banner_link.banners_links_id)
                    ->  Nested Loop  (cost=1050.35..1602.14 rows=1
width=503) (actual time=29.585..9015.077 rows=553 loops=1)
                          ->  Nested Loop Left Join  (cost=1050.35..1593.86
rows=1 width=124) (actual time=29.577..9010.273 rows=553 loops=1)
                                Join Filter: (public.banners_links.id =
users_banners_tot_sub.banner_id)
                                ->  Nested Loop Left Join
(cost=1033.74..1577.21 rows=1 width=116) (actual time=25.904..8738.006
rows=553 loops=1)
                                      Join Filter: (public.banners_links.id
= special_deals.id)
                                      Filter: (special_deals.special_deal IS
NULL)
                                      ->  Nested Loop Left Join
(cost=964.12..1480.67 rows=1 width=108) (actual time=20.905..8259.497
rows=553 loops=1)
                                            Join Filter:
(public.banners_links.id = banners_banner_types.banner_id)
                                            ->  Bitmap Heap Scan on
banners_links  (cost=4.35..42.12 rows=1 width=73) (actual time=0.160..1.122
rows=359 loops=1)
                                                  Recheck Cond:
((merchant_id = 5631) AND (merchant_id = 5631))
                                                  Filter: ((status)::text =
'0'::text)
                                                  ->  Bitmap Index Scan on
banners_links_merchant_id_idx  (cost=0.00..4.35 rows=10 width=0) (actual
time=0.123..0.123 rows=424 loops=1)
                                                        Index Cond:
((merchant_id = 5631) AND (merchant_id = 5631))
                                            ->  Hash Join
(cost=959.77..1432.13 rows=514 width=43) (actual time=0.899..22.685 rows=658
loops=359)
                                                  Hash Cond:
(banners_banner_types.type_id = banner_types.id)
                                                  ->  Hash IN Join
(cost=957.32..1422.52 rows=540 width=16) (actual time=0.897..21.946 rows=658
loops=359)
                                                        Hash Cond:
(banners_banner_types.banner_id = public.banners_links.id)
                                                        ->  Seq Scan on
banners_banner_types  (cost=0.00..376.40 rows=22240 width=16) (actual
time=0.004..10.164 rows=22240 loops=359)
                                                        ->  Hash
(cost=952.02..952.02 rows=424 width=8) (actual time=0.790..0.790 rows=424
loops=1)
                                                              ->  Bitmap
Heap Scan on banners_links  (cost=11.54..952.02 rows=424 width=8) (actual
time=0.108..0.503 rows=424 loops=1)
                                                                    Recheck
Cond: (merchant_id = 5631)
                                                                    ->
Bitmap Index Scan on banners_links_merchant_id_idx  (cost=0.00..11.43
rows=424 width=0) (actual time=0.078..0.078 rows=424 loops=1)

Index Cond: (merchant_id = 5631)
                                                  ->  Hash  (cost=2.20..2.20
rows=20 width=43) (actual time=0.033..0.033 rows=20 loops=1)
                                                        ->  Seq Scan on
banner_types  (cost=0.00..2.20 rows=20 width=43) (actual time=0.004..0.017
rows=20 loops=1)
                                      ->  HashAggregate  (cost=69.62..79.24
rows=769 width=16) (actual time=0.008..0.498 rows=780 loops=553)
                                            ->  Seq Scan on banner_deals
(cost=0.00..53.75 rows=3175 width=16) (actual time=0.004..1.454 rows=3175
loops=1)
                                ->  HashAggregate  (cost=16.61..16.62 rows=1
width=24) (actual time=0.007..0.291 rows=424 loops=553)
                                      ->  Nested Loop  (cost=0.00..16.60
rows=1 width=24) (actual time=0.056..3.123 rows=424 loops=1)
                                            ->  Index Scan using
users_banners_affiliate_id_idx on users_banners  (cost=0.00..8.30 rows=1
width=16) (actual time=0.046..0.555 rows=424 loops=1)
                                                  Index Cond: ((affiliate_id
= 5631) AND (affiliate_id = 5631))
                                                  Filter: ((status)::text =
'3'::text)
                                            ->  Index Scan using
users_banners_id_idx on users_banners_rotation  (cost=0.00..8.29 rows=1
width=16) (actual time=0.003..0.004 rows=1 loops=424)
                                                  Index Cond:
(users_banners_rotation.users_banners_id = users_banners.id)
                          ->  Index Scan using "banners_org_id_banner.idx"
on banners_org  (cost=0.00..8.27 rows=1 width=387) (actual time=0.005..0.006
rows=1 loops=553)
                                Index Cond: (public.banners_links.id =
banners_org.id_banner)
                    ->  Sort  (cost=1876.01..1876.50 rows=194 width=30)
(actual time=0.041..0.161 rows=290 loops=553)
                          Sort Key: CASE WHEN
(precalculated_stats_banners_links.clicks_total > 0) THEN
(((precalculated_stats_banners_links.revenue_total_affiliate /
(precalculated_stats_banners_links.clicks_total)::numeric))::double
precision / 1000::double precision) ELSE 0::double precision END
                          ->  Merge IN Join  (cost=1819.78..1868.64 rows=194
width=30) (actual time=16.769..21.879 rows=290 loops=1)
                                Merge Cond:
(precalculated_stats_banners_links.banners_links_id =
public.banners_links.id)
                                ->  Sort  (cost=849.26..869.24 rows=7993
width=30) (actual time=12.486..15.740 rows=7923 loops=1)
                                      Sort Key:
precalculated_stats_banners_links.banners_links_id
                                      ->  Index Scan using
pre_calc_banners_status on precalculated_stats_banners_links
(cost=0.00..331.13 rows=7993 width=30) (actual time=0.007..6.291 rows=7923
loops=1)
                                            Index Cond: (status = 4)
                                ->  Sort  (cost=970.52..971.58 rows=424
width=8) (actual time=0.879..1.023 rows=366 loops=1)
                                      Sort Key: public.banners_links.id
                                      ->  Bitmap Heap Scan on banners_links
(cost=11.54..952.02 rows=424 width=8) (actual time=0.123..0.509 rows=424
loops=1)
                                            Recheck Cond: (merchant_id =
5631)
                                            ->  Bitmap Index Scan on
banners_links_merchant_id_idx  (cost=0.00..11.43 rows=424 width=0) (actual
time=0.089..0.089 rows=424 loops=1)
                                                  Index Cond: (merchant_id =
5631)
              ->  Seq Scan on fetch_banners  (cost=0.00..2.88 rows=88
width=78) (actual time=0.003..0.042 rows=88 loops=553)
        ->  Hash IN Join  (cost=957.32..1606.24 rows=93 width=16) (actual
time=10.933..10.933 rows=0 loops=553)
              Hash Cond: (reward_ratings.banner_id =
public.banners_links.id)
              ->  Seq Scan on reward_ratings  (cost=0.00..633.66 rows=3822
width=16) (actual time=0.007..8.955 rows=4067 loops=553)
                    Filter: ((now() >= period_start) AND (now() <=
period_end))
              ->  Hash  (cost=952.02..952.02 rows=424 width=8) (actual
time=0.738..0.738 rows=424 loops=1)
                    ->  Bitmap Heap Scan on banners_links
(cost=11.54..952.02 rows=424 width=8) (actual time=0.118..0.475 rows=424
loops=1)
                          Recheck Cond: (merchant_id = 5631)
                          ->  Bitmap Index Scan on
banners_links_merchant_id_idx  (cost=0.00..11.43 rows=424 width=0) (actual
time=0.087..0.087 rows=424 loops=1)
                                Index Cond: (merchant_id = 5631)
Total runtime: 15283.225 ms

If I change 1 of the redundant checks:

/* SUBQUERY banners_links */ (
    SELECT
        *
    FROM
        banners_links
    WHERE
        merchant_id = 5631
) AS banners_links

into just banner_links, PG comes up with the (large) plan I posted earlier.

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