Как индексировать SQL с несколькими условиями И, вложенными в ИЛИ

Хочу ускорить следующий sql (стоимость 19685.75). Могу ли я проиндексировать этот sql, который имеет несколько сложных вложенных условий AND, сочетающихся с OR в выражении WHERE?

SELECT DISTINCT
    ON ("crawler_url"."url") U0."id"
FROM "characteristics_text" U0 LEFT OUTER
JOIN "characteristics_text_urls"
    ON (U0."id" = "characteristics_text_urls"."text_id") LEFT OUTER
JOIN "crawler_url"
    ON ("characteristics_text_urls"."url_id" = "crawler_url"."id")
WHERE ( 
    (
        U0."publication_date" BETWEEN '2018-01-01' AND '2018-12-31'
        AND EXTRACT('month' FROM U0."publication_date") = 10
    )
        OR 
    (
        U0."publication_date" IS NULL
        AND U0."lastmod" BETWEEN '2018-01-01' AND '2018-12-31'
        AND EXTRACT('month' FROM U0."lastmod") = 10
    )
        OR 
    (
        U0."publication_date" IS NULL
        AND U0."lastmod" IS NULL
        AND U0."created_at" BETWEEN '2018-01-01 00:00:00+08:00' AND '2018-12-31 23:59:59.999999+08:00'
        AND EXTRACT('month' FROM U0."created_at" AT TIME ZONE 'Asia/Hong_Kong') = 10
    )
        OR 
    (
        U0."publication_date" >= '2018-08-01'
        AND U0."publication_date" < '2018-10-31'
    )
        OR 
    (
        U0."publication_date" IS NULL
        AND U0."lastmod" >='2018-08-01'
        AND U0."lastmod" < '2018-10-31'
    )
        OR 
    (
        U0."publication_date" IS NULL
        AND U0."lastmod" IS NULL
        AND U0."created_at" >= '2018-07-31 16:00:00+00:00'
        AND U0."created_at" < '2018-10-30 16:00:00+00:00'
    ) 
)
ORDER BY  "crawler_url"."url" ASC, U0."created_at" DESC

Текст таблицы содержит следующие поля и индексы (некоторые другие поля не показаны)

                                            Table "public.characteristics_text"                                                                                                     
            Column         |           Type           |                             Modifiers                                                                                        
    ------------------------+--------------------------+-------------------------------------------------------------------                                                           
    id                     | integer                  | not null default nextval('characteristics_text_id_seq'::regclass)
    text                   | text                     | 
    created_at             | timestamp with time zone | not null
    lastmod                | date                     | 
    publication_date       | date                     | 
    Indexes:
        "characteristics_text_pkey" PRIMARY KEY, btree (id)
        "characteristics_text_fde81f11" btree (created_at)
        "characteristics_text_lastmod_3bff34c2_uniq" btree (lastmod)
        "characteristics_text_publication_date_772c1bda_uniq" btree (publication_date)
        "characteristics_text_publication_date_c6311385_idx" btree (publication_date, lastmod, created_at)

Я добавил три отдельных индекса для created_at, lastmod и publish_date; и один индекс с несколькими столбцами для этих полей.

Но в запросе postgres EXPAIN это предложение where по-прежнему использует Seq Scan, но не Index Scan.

->  Seq Scan on characteristics_text u0  (cost=0.00..19685.75 rows=14535 width=12)
    Filter: (
            (
                (publication_date >= '2018-01-01'::date) AND 
                (publication_date <= '2018-12-31'::date) AND 
                (
                        date_part(
                            'month'::text, (publication_date)::timestamp without time zone
                ) = 10::double precision)
            ) OR 

                ((publication_date IS NULL) AND (lastmod >= '2018-01-01'::date) AND (lastmod <= '2018-12-31'::date) AND (date_part('month'::text, (lastmod)::timestamp without time zone) = 10::double precision)) OR ((publication_date IS NULL) AND (lastmod IS NULL) AND (created_at >= '2017-12-31 16:00:00+00'::timestamp with time zone) AND (created_at <= '2018-12-31 15:59:59.999999+00'::timestamp with time zone) AND (date_part('month'::text, timezone('Asia/Hong_Kong'::text, created_at)) = 10::double precision)) OR ((publication_date >= '2018-08-01'::date) AND (publication_date < '2018-10-31'::date)) OR ((publication_date IS NULL) AND (lastmod >= '2018-08-01'::date) AND (lastmod < '2018-10-31'::date)) OR ((publication_date IS NULL) AND (lastmod IS NULL) AND (created_at >= '2018-07-31 16:00:00+00'::timestamp with time zone) AND (created_at < '2018-10-30 16:00:00+00'::timestamp with time zone))
)

Мои вопросы:
1. Можно ли заставить postgres использовать сканирование индекса для этого сложного предложения SELECT?
2. Нужно ли мне создавать один индекс с несколькими столбцами для каждого предложения AND? Например создание индекса (publication_date, lastmod) в чем причина?

    (
        U0."publication_date" IS NULL
        AND U0."lastmod" BETWEEN '2018-01-01' AND '2018-12-31'
        AND EXTRACT('month' FROM U0."lastmod") = 10
    )

3. Работает ли индекс при поиске IS NULL? Должен ли поиск поля для IS NULL быть индексом?

ОБНОВЛЕНО 4 ноября 2018 г.

Когда я пытаюсь минимизировать запрос, проверяя поля одно за другим, поля publication_date и last_mod запускают сканирование индекса по отдельности, а created_at не может:

Это потому, что created_at - это временная метка? Но почему индекс не работает для метки времени?

explain SELECT DISTINCT
    ON ("crawler_url"."url") U0."id"
FROM "characteristics_text" U0 LEFT OUTER
JOIN "characteristics_text_urls"
    ON (U0."id" = "characteristics_text_urls"."text_id") LEFT OUTER
JOIN "crawler_url"
    ON ("characteristics_text_urls"."url_id" = "crawler_url"."id")
WHERE ( 
(
        U0."created_at" BETWEEN '2018-01-01 00:00:00+08:00' AND '2018-12-31 23:59:59.999999+08:00'
        AND EXTRACT('month' FROM U0."created_at" AT TIME ZONE 'Asia/Hong_Kong') = 10
    )   
)
ORDER BY  "crawler_url"."url" ASC, U0."created_at" DESC



Unique  (cost=18004.05..18006.01 rows=393 width=86)
->  Sort  (cost=18004.05..18005.03 rows=393 width=86)
        Sort Key: crawler_url.url, u0.created_at
        ->  Nested Loop Left Join  (cost=0.71..17987.11 rows=393 width=86)
            ->  Nested Loop Left Join  (cost=0.42..17842.25 rows=393 width=16)
                    ->  Seq Scan on characteristics_text u0  (cost=0.00..15467.37 rows=393 width=12)
                        Filter: ((created_at >= '2017-12-31 16:00:00+00'::timestamp with time zone) AND (created_at <= '2018-12-31 15:59:59.999999+00'::timestamp with time zone) AND (date_part('month'::text, timezone('Asia/Hong_Kong'::text, created_at)) = 10::double precision))
                    ->  Index Scan using characteristics_text_urls_65eb77fe on characteristics_text_urls  (cost=0.42..6.03 rows=1 width=8)
                        Index Cond: (u0.id = text_id)
            ->  Index Scan using crawler_url_pkey on crawler_url  (cost=0.29..0.36 rows=1 width=78)
                    Index Cond: (characteristics_text_urls.url_id = id)

publication_date кажется запускает сканирование индекса:

(
    U0."publication_date" IS NULL
    AND U0."lastmod" >='2018-08-01'
    AND U0."lastmod" < '2018-10-31'
)


Unique  (cost=17053.26..17085.63 rows=6473 width=86)
->  Sort  (cost=17053.26..17069.44 rows=6473 width=86)
        Sort Key: crawler_url.url, u0.created_at
        ->  Nested Loop Left Join  (cost=11130.73..16643.51 rows=6473 width=86)
            ->  Hash Right Join  (cost=11130.44..14257.63 rows=6473 width=16)
                    Hash Cond: (characteristics_text_urls.text_id = u0.id)
                    ->  Seq Scan on characteristics_text_urls  (cost=0.00..1858.01 rows=120601 width=8)
                    ->  Hash  (cost=11049.53..11049.53 rows=6473 width=12)
                        ->  Bitmap Heap Scan on characteristics_text u0  (cost=186.95..11049.53 rows=6473 width=12)
                                Recheck Cond: ((publication_date IS NULL) AND (lastmod >= '2018-08-01'::date) AND (lastmod < '2018-10-31'::date))
                                ->  Bitmap Index Scan on characteristics_text_publication_date_c6311385_idx  (cost=0.00..185.33 rows=6473 width=0)
                                    Index Cond: ((publication_date IS NULL) AND (lastmod >= '2018-08-01'::date) AND (lastmod < '2018-10-31'::date))
            ->  Index Scan using crawler_url_pkey on crawler_url  (cost=0.29..0.36 rows=1 width=78)
                    Index Cond: (characteristics_text_urls.url_id = id)