The optimization process and recommendations:

1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
2. Use Numeric Column Types For Numeric Values (query line: 18): Referencing a numeric value (e.g. 104026) as a string in a WHERE clause might result in poor performance. Possible impacts of storing numbers as varchars: more space will be used, you won't be able to perform arithmetic operations, the data won't be self-validated, aggregation functions like SUM won't work, the output may sort incorrectly and more. If the column is numeric, remove the quotes from the constant value, to make sure a numeric comparison is done.

Optimal indexes for this query:

CREATE INDEX order_items_idx_deleted_at_order_id_sku ON "order_items" ("deleted_at","order_id","sku");
CREATE INDEX orders_idx_country_deleted_at ON "orders" ("country","deleted_at");

The optimized query:

SELECT
        "orders".id,
        "orders".country,
        "orders".total_items,
        "orders".total_weight,
        "orders".batch_id 
    FROM
        "orders" 
    WHERE
        EXISTS (
            SELECT
                * 
            FROM
                "order_items" 
            WHERE
                "orders"."id" = "order_items"."order_id" 
                AND "order_items"."sku" IN (
                    '104026'
                ) 
                AND "order_items"."deleted_at" IS NULL
        ) 
        AND "orders"."country" = 'Germany' 
        AND "orders"."deleted_at" IS NULL