…rmance when querying multiple times on postgres (#14315)

1. Fixes handling of `$exists` when querying a field nested to a join
field.
2. Avoids adding new SQL joins when querying a join field multiple times
before
```sql
SELECT DISTINCT
    "categories"."id",
    "categories"."created_at",
    "categories"."created_at"
FROM
    "categories"
    LEFT JOIN "posts_rels" AS "c40660ed_b4bf_4644_bbd1_b7225045bb92"
        ON (
            "categories"."id" = "c40660ed_b4bf_4644_bbd1_b7225045bb92"."categories_id"
            AND "c40660ed_b4bf_4644_bbd1_b7225045bb92"."path" LIKE ?
        )
    LEFT JOIN "posts" AS "6702692c_e77e_41b3_ab0c_038d8ca584f6"
        ON "c40660ed_b4bf_4644_bbd1_b7225045bb92"."parent_id" = "6702692c_e77e_41b3_ab0c_038d8ca584f6"."id"
    LEFT JOIN "posts" AS "b8d97c7b_51c7_448e_96dd_567ac0a24ccf"
        ON "c40660ed_b4bf_4644_bbd1_b7225045bb92"."parent_id" = "b8d97c7b_51c7_448e_96dd_567ac0a24ccf"."id"
    LEFT JOIN "posts" AS "89586995_3641_4226_b047_dc3c96b33d9f"
        ON "c40660ed_b4bf_4644_bbd1_b7225045bb92"."parent_id" = "89586995_3641_4226_b047_dc3c96b33d9f"."id"
WHERE
    "6702692c_e77e_41b3_ab0c_038d8ca584f6"."title" = ?
    AND "b8d97c7b_51c7_448e_96dd_567ac0a24ccf"."title" IS NOT NULL
    AND "89586995_3641_4226_b047_dc3c96b33d9f"."is_filtered" = ?
ORDER BY
    "categories"."created_at" DESC
LIMIT ?
-- params: ["categories", "my-title", 1, 10]
```
now
```sql
SELECT DISTINCT
    "categories"."id",
    "categories"."created_at",
    "categories"."created_at"
FROM
    "categories"
    LEFT JOIN "posts_rels" AS "b6ba04a3_a557_4321_abfb_7b58250baa2c"
        ON (
            "categories"."id" = "b6ba04a3_a557_4321_abfb_7b58250baa2c"."categories_id"
            AND "b6ba04a3_a557_4321_abfb_7b58250baa2c"."path" LIKE ?
        )
    LEFT JOIN "posts" AS "86c9090f_c50b_485d_a9bb_1ca8b5ad079d"
        ON "b6ba04a3_a557_4321_abfb_7b58250baa2c"."parent_id" = "86c9090f_c50b_485d_a9bb_1ca8b5ad079d"."id"
WHERE
    "86c9090f_c50b_485d_a9bb_1ca8b5ad079d"."title" = ?
    AND "86c9090f_c50b_485d_a9bb_1ca8b5ad079d"."title" IS NOT NULL
    AND "86c9090f_c50b_485d_a9bb_1ca8b5ad079d"."is_filtered" = ?
ORDER BY
    "categories"."created_at" DESC
LIMIT ?
-- params: ["categories", "my-title", 1, 10]
```