Ko te DBMS mo tenei kaupapa e mahi ana i runga i nga kaupapa kotahi - "сказали копать, я и копаю". Kaore e taea e to tono te whakaroa noa i nga mahi tata, me te tango i nga rauemi tukatuka, engari ka "whakataka" te katoa o te papaa raraunga, "kai" nga mahara katoa e waatea ana. te whakamarumaru ki te recursion mutunga kore - te kawenga a te kaiwhakawhanake ake.
I roto i te PostgreSQL, te kaha ki te whakamahi i nga patai recursive ma WITH RECURSIVE
Kaua e tuhia nga patai recursive
A ka tuhi i nga mea kore-recursive. Tena koe, To K.O.
Inaa, he maha nga mahi a PostgreSQL ka taea e koe te whakamahi e kore tono hokinga.
Whakamahia he huarahi tino rereke ki te raruraru
I etahi wa ka taea e koe te titiro noa ki te raruraru mai i te "taha rereke". I hoatu e ahau he tauira o taua ahuatanga i roto i te tuhinga
WITH RECURSIVE src AS (
SELECT '{2,3,5,7,11,13,17,19}'::integer[] arr
)
, T(i, val) AS (
SELECT
1::bigint
, 1
UNION ALL
SELECT
i + 1
, val * arr[i]
FROM
T
, src
WHERE
i <= array_length(arr, 1)
)
SELECT
val
FROM
T
ORDER BY -- отбор финального результата
i DESC
LIMIT 1;
Ka taea te whakakapi i tenei tono ki tetahi whiringa mai i nga tohunga pangarau:
WITH src AS (
SELECT unnest('{2,3,5,7,11,13,17,19}'::integer[]) prime
)
SELECT
exp(sum(ln(prime)))::integer val
FROM
src;
Whakamahia te whakaputa_raupapa hei utu mo nga koropiko
Me kii kei te anga tatou ki te mahi ki te whakaputa i nga tohu tuatahi mo te aho 'abcdefgh'
:
WITH RECURSIVE T AS (
SELECT 'abcdefgh' str
UNION ALL
SELECT
substr(str, 1, length(str) - 1)
FROM
T
WHERE
length(str) > 1
)
TABLE T;
Kei te tino hiahia koe ki te whakahoki mai i konei?.. Ki te whakamahi koe LATERAL
и generate_series
, ka kore koe e hiahia CTE:
SELECT
substr(str, 1, ln) str
FROM
(VALUES('abcdefgh')) T(str)
, LATERAL(
SELECT generate_series(length(str), 1, -1) ln
) X;
Hurihia te hanganga pātengi raraunga
Hei tauira, kei a koe he ripanga o nga karere huinga me nga hononga mai i a wai i whakautu ki a wai, he miro ranei kei roto
CREATE TABLE message(
message_id
uuid
PRIMARY KEY
, reply_to
uuid
REFERENCES message
, body
text
);
CREATE INDEX ON message(reply_to);
Ana, he ahua penei te ahua o te tono mo te tango i nga karere katoa mo te kaupapa kotahi:
WITH RECURSIVE T AS (
SELECT
*
FROM
message
WHERE
message_id = $1
UNION ALL
SELECT
m.*
FROM
T
JOIN
message m
ON m.reply_to = T.message_id
)
TABLE T;
Engari i te mea ka hiahia tonu tatou i te kaupapa katoa mai i te karere pakiaka, he aha tatou ka kore ai tāpirihia tana ID ki ia urunga aunoa?
-- добавим поле с общим идентификатором темы и индекс на него
ALTER TABLE message
ADD COLUMN theme_id uuid;
CREATE INDEX ON message(theme_id);
-- инициализируем идентификатор темы в триггере при вставке
CREATE OR REPLACE FUNCTION ins() RETURNS TRIGGER AS $$
BEGIN
NEW.theme_id = CASE
WHEN NEW.reply_to IS NULL THEN NEW.message_id -- берем из стартового события
ELSE ( -- или из сообщения, на которое отвечаем
SELECT
theme_id
FROM
message
WHERE
message_id = NEW.reply_to
)
END;
RETURN NEW;
END;
$$ LANGUAGE plpgsql;
CREATE TRIGGER ins BEFORE INSERT
ON message
FOR EACH ROW
EXECUTE PROCEDURE ins();
Inaianei ka taea te whakaiti i a maatau patai recursive ki tenei:
SELECT
*
FROM
message
WHERE
theme_id = $1;
Whakamahia "whakatika"
Mena kaore e taea e tatou te whakarereke i te hanganga o te papaaarangi mo etahi take, kia kite tatou he aha ta tatou e whakawhirinaki ai na te mea ko te hapa i roto i nga raraunga kaore e arahi ki te hokinga mutunga kore.
He porotiti hohonu recursion
Ka whakanuia noa e matou te porotiti ma te kotahi i ia taahiraa whakamuri kia tae ra ano ki te tepe e whakaaro ana matou he koretake:
WITH RECURSIVE T AS (
SELECT
0 i
...
UNION ALL
SELECT
i + 1
...
WHERE
T.i < 64 -- предел
)
Pro: Ka ngana ana tatou ki te kopikopiko, ka kore e neke ake i te tepe o nga whitiwhitinga "hohonu".
raruraru: Karekau he taurangi ka kore matou e whakahaere ano i taua rekoata - hei tauira, i te hohonutanga o te 15 me te 25, katahi ka +10. A kaore tetahi i oati i tetahi mea mo te "whanui".
I runga i nga tikanga, kare he mutunga o taua recursion, engari mena ka piki haere te maha o nga rekoata i ia taahiraa, ka tino mohio tatou me pehea te mutunga...
Kaitiaki o te "ara"
Ka taapirihia e matou nga kaitautuhi ahanoa katoa i tutaki ki a matou i te ara whakamuri ki roto i tetahi huinga, he "ara" ahurei ki reira:
WITH RECURSIVE T AS (
SELECT
ARRAY[id] path
...
UNION ALL
SELECT
path || id
...
WHERE
id <> ALL(T.path) -- не совпадает ни с одним из
)
Pro: Mena he huringa kei roto i nga raraunga, kare rawa matou e tukatuka i te rekoata kotahi i roto i te huarahi kotahi.
raruraru: Engari i te wa ano, ka taea e tatou te karo i nga rekoata katoa me te kore e whakahoki ano.
Tepete roa o te ara
Hei karo i te ahuatanga o te recursion "kopikopiko" i te hohonu e kore e taea te mohio, ka taea e tatou te whakakotahi i nga tikanga e rua o mua. Ranei, ki te kore matou e hiahia ki te tautoko i nga mara koretake, taapirihia te tikanga mo te haere tonu o te hokinga mai me te whakatau tata mo te roa o te huarahi:
WITH RECURSIVE T AS (
SELECT
ARRAY[id] path
...
UNION ALL
SELECT
path || id
...
WHERE
id <> ALL(T.path) AND
array_length(T.path, 1) < 10
)
Whiriwhiria he tikanga ki to reka!
Source: will.com