\timing
with j as (
SELECT
$$
    {"header":{"vendor":"Target","date":"10/12/2017","instrument":"Discover Card","module":"hdrio","total":47.74,"location":"Stow, OH","transaction":"purchase","offset":"dcard"},"item":[{"vend item":"HERBAL","amt":7.99,"account":"home supplies","item":"shampoo","reason":"hygiene"},{"vend item":"HERBAL","amt":7.99,"account":"home supplies","item":"conditioner","reason":"hygiene"},{"vend item":"BUILDING SET","amt":28.74,"account":"recreation","item":"legos","reason":"toys","qty":6,"uom":"ea"},{"vend item":"OH TAX","amt":3.02,"account":"sales tax","item":"sales tax","reason":"sales tax","rate":"0.0675"}]}
$$::jsonb jb
)
,os AS (
    SELECT
        flag, 
        sign,
        x.offs
    FROM
        j
        JOIN LATERAL
        (
            VALUES
            ('ITEM',1,null),
            ('OFFSET',-1,j.jb->'header'->>'offset')
        ) x (flag, sign, offs) ON TRUE
)
,build AS (
SELECT
    array['item',rn::text]::text jpath
    ,COALESCE(os.offs,ae.e->>'account') acct
    ,(ae.e->>'amt')::numeric * os.sign amount
FROM
    j
    LEFT JOIN LATERAL JSONB_ARRAY_ELEMENTS(J.JB->'item') WITH ORDINALITY ae(e,rn) ON TRUE
    CROSS JOIN os
ORDER BY
    ae.rn ASC,
    os.flag ASC
)
,agg AS (
SELECT
    jsonb_build_object('gl',jsonb_agg(row_to_json(b))) gl
FROM
    build b
)
SELECT
    jsonb_pretty(agg.gl||j.jb)
FROM
    agg
    CROSS JOIN j