在视图中，最好将ID推送到两个CTE中，以获得完整的性能增益。如果没有其他答案，我们将在一两天内接受。谢谢。@Matt为什么不能在视图中重复使用？postgresql不允许在视图中使用，大多数平台都允许。只需去掉cte中的where，从某个角度来看，这将是

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在视图中，最好将ID推送到两个CTE中，以获得完整的性能增益。如果没有其他答案，我们将在一两天内接受。谢谢。@Matt为什么不能在视图中重复使用？postgresql不允许在视图中使用，大多数平台都允许。只需去掉cte中的where，从某个角度来看，这将是很好的——您仍然应该获得性能改进。

SELECT *
FROM test.tPurchases AS tPurchases
INNER JOIN test.tLicences
    ON tLicences.purchase_id = tPurchases.id
LEFT JOIN (
    SELECT
        purchase_id,
        SUM(
            CASE
                assignment_newer_id IS NOT null
                WHEN true THEN 1
                WHEN false THEN 0
            END
        ) AS prchs_quantity_assigned,
        SUM(
            CASE
                assignment_newer_id IS null AND
                current_timestamp
                    BETWEEN licence_availability_start AND
                        licence_availability_end
                WHEN true THEN 1
                WHEN false THEN 0
            END
        ) AS prchs_quantity_notAssignedAndCanBeAssigned,
        SUM(
            CASE
                assignment_newer_id IS null AND
                current_timestamp < licence_availability_start
                WHEN true THEN 1
                WHEN false THEN 0
            END
        ) AS prchs_quantity_notAssignedAndCannotBeAssigned
    FROM (
        SELECT
            tPurchases.id AS purchase_id,
            tPurchases.date_ AS purchase_date,
            tLicences.id AS licence_id,
            GREATEST(
                tPurchases.date_,
                older.end_,
                older.start + '1 day'::interval
            ) AS licence_availability_start,
            CASE
                WHEN newer.id IS NULL THEN 'infinity'
                ELSE newer.start - '1 day'::interval
            END AS licence_availability_end,
            COALESCE(newer.start, 'infinity') AS licence_availability_uninstallBy,
            older.id AS assignment_older_id,
            older.start AS assignment_older_start,
            older.end_ AS assignment_older_end,
            newer.id AS assignment_newer_id,
            newer.start AS assignment_newer_start,
            newer.end_ AS assignment_newer_end
        FROM test.tLicences
        INNER JOIN test.tPurchases
            ON tPurchases.id = tLicences.purchase_id
        LEFT JOIN test.tAssignments AS older
            ON (
                NOT older.deleted AND
                older.licence_id = tLicences.id
            )
        LEFT JOIN test.tAssignments AS newer
            ON (
                NOT newer.deleted AND
                newer.id <> older.id AND
                newer.licence_id = older.licence_id
            )
        WHERE
            NOT tLicences.deleted
        UNION
        SELECT
            tPurchases.id AS purchase_id,
            tPurchases.date_ AS purchase_date,
            tLicences.id AS licence_id,
            tPurchases.date_ AS licence_availability_start,
            oldest.start - '1 day'::interval AS licence_availability_end,
            oldest.start AS licence_availability_uninstallBy,
            null AS assignment_older_id,
            null AS assignment_older_start,
            null AS assignment_older_end,
            oldest.id AS assignment_newer_id,
            oldest.start AS assignment_newer_start,
            oldest.end_ AS assignment_newer_end
        FROM test.tLicences
        INNER JOIN test.tPurchases
            ON tPurchases.id = tLicences.purchase_id
        INNER JOIN test.tAssignments AS oldest
            ON oldest.licence_id = tLicences.id
        WHERE
            NOT tLicences.deleted AND
            NOT oldest.deleted
    ) AS periodsOfAvailability_start
    WHERE
        (assignment_newer_id IS null OR assignment_newer_end IS null)
    GROUP BY purchase_id
) AS purchase_quantities_assignnments
    ON
        purchase_quantities_assignnments.purchase_id = tPurchases.id
WHERE
    tLicences.id = 19 /* [Unexpected behaviour] The full set for "purchase_quantities_assignnments" is generated */
    --tLicences.id = 19 AND tLicences.purchase_id = ? /* [Expected behaviour] Only the single relevant row for "purchase_quantities_assignnments" appears to be generated */

    --tLicences.id = 19 AND tPurchases.id = ? /* [Expected behaviour] Only the single relevant row for "purchase_quantities_assignnments" appears to be generated */
    --tLicences.purchase_id = ? /* [Expected behaviour] Only the single relevant row for "purchase_quantities_assignnments" appears to be generated. Note: This is a different query *result* than the others */

CREATE SCHEMA test;

CREATE TABLE test.tPurchases (
    id                  serial      not null,
    date_               date        not null,               
    /* … */
    deleted             boolean     not null    DEFAULT false,
    PRIMARY KEY (id)
);

CREATE TABLE test.tLicences (
    id                  serial      not null,
    purchase_id         integer     not null,
    /* … */
    deleted             boolean     not null    DEFAULT false,
    PRIMARY KEY (id),
    FOREIGN KEY (purchase_id)
        REFERENCES test.tPurchases (id)
        ON UPDATE RESTRICT
        ON DELETE RESTRICT
);
CREATE INDEX ON test.tLicences(purchase_id);

CREATE TABLE test.tAssignments (
    id                  serial      not null,
    licence_id          integer     not null,
    start               date        not null,
    end_                date,
    /* … */
    deleted             boolean     not null    DEFAULT false,
    PRIMARY KEY (id),
    FOREIGN KEY (licence_id)
        REFERENCES test.tLicences (id)
        ON UPDATE RESTRICT
        ON DELETE RESTRICT,
    CHECK (start <= end_)
);
CREATE INDEX ON test.tAssignments(licence_id);



INSERT INTO test.tPurchases(id, date_)
    SELECT
        id,
        '2000-01-01'::timestamp +  random() * '1 year'::interval AS date_
    FROM generate_series(1, 10) AS id
;

INSERT INTO test.tLicences(id, purchase_id, deleted)
    SELECT
        id,
        trunc(random() * 10 + 1) AS purchase_id,
        (random() > 0.99) AS deleted
    FROM generate_series(1, 30000) AS id
;

INSERT INTO test.tAssignments(id, licence_id, start, end_, deleted)
    SELECT
        assignments.id,
        assignments.licence_id,
        tPurchases.date_ + ((rank * 20 + random() * 10) || ' days')::interval AS start,
        CASE
            assignments.rank = max(assignments.rank) OVER (PARTITION BY assignments.licence_id) AND
            random() > 0.5
            WHEN true THEN null
            ELSE tPurchases.date_ + ((rank * 20 + 10 + random() * 10) || ' days')::interval
        END AS end_,
        (random() > 0.95) AS deleted
    FROM (
        SELECT
            assignments.id,
            assignments.licence_id,
            rank() OVER (PARTITION BY assignments.licence_id ORDER BY assignments.id) AS rank
        FROM (
            SELECT
                id,
                trunc(random() * 30000 + 1) AS licence_id
            FROM generate_series(1, 100000) AS id
        ) AS assignments
    ) AS assignments
    INNER JOIN test.tLicences
        ON tLicences.id = assignments.licence_id
    INNER JOIN test.tPurchases
        ON tPurchases.id = tLicences.purchase_id
;

WITH myPurchases AS
( 
  SELECT *
  FROM test.tPurchases AS tPurchases
  WHERE tLicences.id = 19 
), periodsOfAvailability_start AS
(
  SELECT
      tPurchases.id AS purchase_id,
      tPurchases.date_ AS purchase_date,
      tLicences.id AS licence_id,
      GREATEST(tPurchases.date_, older.end_, older.start + '1 day'::interval) AS licence_availability_start,
      CASE WHEN newer.id IS NULL THEN 'infinity' ELSE newer.start - '1 day'::interval END AS licence_availability_end,
      COALESCE(newer.start, 'infinity') AS licence_availability_uninstallBy,
      older.id AS assignment_older_id,
      older.start AS assignment_older_start,
      older.end_ AS assignment_older_end,
      newer.id AS assignment_newer_id,
      newer.start AS assignment_newer_start,
      newer.end_ AS assignment_newer_end
  FROM test.tLicences
  INNER JOIN myPurchases AS tPurchases ON tPurchases.id = tLicences.purchase_id
  LEFT JOIN test.tAssignments AS older ON (NOT older.deleted AND older.licence_id = tLicences.id)
  LEFT JOIN test.tAssignments AS newer ON (NOT newer.deleted AND newer.id <> older.id AND newer.licence_id = older.licence_id)
  WHERE NOT tLicences.deleted

  UNION

  SELECT
      tPurchases.id AS purchase_id,
      tPurchases.date_ AS purchase_date,
      tLicences.id AS licence_id,
      tPurchases.date_ AS licence_availability_start,
      oldest.start - '1 day'::interval AS licence_availability_end,
      oldest.start AS licence_availability_uninstallBy,
      null AS assignment_older_id,
      null AS assignment_older_start,
      null AS assignment_older_end,
      oldest.id AS assignment_newer_id,
      oldest.start AS assignment_newer_start,
      oldest.end_ AS assignment_newer_end
  FROM test.tLicences
  INNER JOIN myPurchases AS tPurchases ON tPurchases.id = tLicences.purchase_id
  INNER JOIN test.tAssignments AS oldest ON oldest.licence_id = tLicences.id
  WHERE NOT tLicences.deleted AND NOT oldest.deleted
), purchase_quantities_assignnments AS
(
  SELECT
    purchase_id,
    SUM(CASE WHEN assignment_newer_id IS NOT null THEN 1 ELSE 0 END) AS prchs_quantity_assigned,
    SUM(CASE WHEN assignment_newer_id IS null AND current_timestamp BETWEEN licence_availability_start AND licence_availability_end THEN 1 ELSE false END) AS prchs_quantity_notAssignedAndCanBeAssigned,
    SUM(CASE WHEN assignment_newer_id IS null AND current_timestamp < licence_availability_start THEN 1 ELSE 0 END) AS prchs_quantity_notAssignedAndCannotBeAssigned
  FROM periodsOfAvailability_start
  WHERE assignment_newer_id IS null OR assignment_newer_end IS null
  GROUP BY purchase_id
)
SELECT *
FROM myPurchases AS tPurchases
INNER JOIN test.tLicences ON tLicences.purchase_id = tPurchases.id
LEFT JOIN purchase_quantities_assignnments ON purchase_quantities_assignnments.purchase_id = tPurchases.id