Como simplificar uma consulta de seleção que contém muitas seleções internas e aumentar o desempenho no PostgreSQL

1. Receba todos os cálculos agregados em uma única seleção - agrupe por consulta.
2. Substituir subseleções por junção à esquerda
3. Restrição adicional na consulta interna com base em condições externas.

with gtab4_5 as (
    select 
        gtab04.Productid, gtab05.BatchId
    FROM
       gtab04 
    LEFT JOIN gtab05 
        ON gtab04.Productid = gtab05.Productid  
    WHERE (gtab04.Masked = False AND gtab04.Banned = false)  
        AND  gtab04.patentid in  (321, 313 , 267 , 431) 
)
  SELECT 
   gtab04.Productid,
   gtab04.Product,
   gtab04.SaleUnit ,
   gtab04.Packing,
   gtab04.ConvFact,
   gtab04.PTR,
   gtab04.MRP, 
   gtab04.PRate,
   gtab04.PTR1,
   gtab04.PTR2,
   gtab04.Location,
   0 As ManufId,
   gtab07.PatentId,
   gtab07.Patent,
   gtab07.PatentCd,
   gtab15.TaxId,
   gtab15.TaxName, 
   gtab15.TaxType,
   gtab15.TaxRate, 
   gtab15.TxOMRP,
   ''::text As Manufacture,
   stats_ags.StkEntered AS StkEntered,
   stats_ags.OpeningInQty as OpeningInQty,
   stats_ags.OpeningOutQty as OpeningOutQty,
   0 AS PurchQty, 
   0 AS SRetQty, 
   0 AS PerInQty, 
   0 AS SaleQty, 
   0 AS StockInQty,
   0 AS StockOutQty, 
   0 AS SaleAmt, 
   0 AS DamageQty, 
   0 AS PRetQty, 
   0 AS PerOutQty,
   stats_ags.PrMthSaleQty as PrMthSaleQty,
   stats_ags.PrMthSaleAmt as PrMthSaleAmt,
   gtab04.LandCost,
   gtab05.PTR AS BatchPTR,
      (case when (
      gtab05.Fqty -  gtab05.FIQty)>0 then (gtab05.Fqty -  gtab05.FIQty) else 0 end) as 
   IssdFreeQty, 
   gtab05.MRP AS BatchMRP, 
   gtab05.PRate AS BatchPRate, 
   gtab04.StdPack,  
      (date_part('day',  (Select Min(Expiry) From gtab05 WHERE  gtab05.Productid =    
      gtab04.Productid And gtab05.Qty > gtab05.IQty)-Cast('2014-06-20' AS    
      timestamp))) AS 
   ExpDys, 
   stats_ags.LastSaleDate as LastSaleDate ,
   GTAB05.BatchId, 
   0 AS FreeSaleQty, 
   0 AS ReplSaleQty 
FROM
   gtab04 
INNER JOIN  gtab15 
    ON gtab04.TaxId = gtab15.TaxId  
LEFT JOIN gtab05 
    ON gtab04.Productid = gtab05.Productid   
INNER JOIN gtab07 
    ON gtab07.Patentid = gtab04.Patentid  
left join (
        SELECT gtab10_9.Productid, gtab10_9.BatchId, 
            max( case 
                when gtab10_9.acyrid = 7 
                then gtab10_9.Productid else null 
            end) as StkEntered,
            SUM( case 
                when gtab10_9.acyrid = 7 and ( 
                    gtab10_9.vrid = 6 
                or (
                    gtab10_9.vrid in (10,11,23,42,35) 
                    AND gtab10_9.trdate < Cast('2014-06-01' AS timestamp) 
                ) ) 
                then gtab10_9.qty else 0 
            end) as OpeningInQty,
            SUM( case 
                when gtab10_9.acyrid = 7 
                    and gtab10_9.vrid in( 12,32,33,44 ,45 ,46, 47 ,48 , 49,18 , 34 ,25,27 ,15,26,24 , 43 ,36) 
                    AND gtab10_9.trdate < Cast('2014-06-01'   AS timestamp)                 
                then gtab10_9.qty else 0 
            end) as OpeningOutQty,
            SUM( case 
                when gtab10_9.acyrid = 7 
                    and gtab10_9.vrid in(12 ,32 ,33 ,44 ,45 ,46 ,47,48,49) 
                    AND gtab10_9.trdate BETWEEN '2014-05-01' AND '2014-05-31'               
                then gtab10_9.qty else 0 
            end) as PrMthSaleQty,
            SUM( case 
                when gtab10_9.acyrid = 7 
                    and gtab10_9.vrid in( 12, 32 , 33 , 44 ,45 ,46 , 47,48 , 49) 
                    and (gtab10_9.FreeOrRpl = 0 OR gtab10_9.FreeOrRpl = 2) 
                    AND gtab10_9.trdate BETWEEN '2014-05-01' AND '2014-05-31'               
                then gtab10_9.qty * gtab10_9.ptr else 0 
            end) as PrMthSaleAmt,
            MAX( case 
                when gtab10_9.acyrid = 7 
                    and gtab10_9.VrId in (12,32,33,44,45,46, 47,48,49)
                    and (gtab10_9.FreeOrRpl = 0 OR gtab10_9.FreeOrRpl = 2) 
                then gtab10_9.TrDate else null 
            end) as LastSaleDate
        FROM (
            SELECT gtab10.*, gtab09.*
            FROM gtab10 
            INNER JOIN gtab09 ON gtab09.TranId = gtab10.TranId 
            inner join gtab4_5 on   gtab4_5.Productid = gtab10.Productid and gtab4_5.BatchId = gtab10.BatchId
        ) gtab10_9 
        group by gtab10_9.Productid, gtab10_9.BatchId
    ) stats_ags
    on stats_ags.Productid = gtab04.Productid 
    and stats_ags.BatchId = gtab05.BatchId
WHERE (gtab04.Masked = False AND gtab04.Banned = false)  
    AND  gtab04.patentid in  (321, 313 , 267 , 431) 
ORDER BY Patent, gtab04.Product

Sort  (cost=82928.96..82931.84 rows=1152 width=306) (actual time=447.433..450.191 rows=2421 loops=1)
  Sort Key: gtab07.patent, gtab04.product
  Sort Method: external merge  Disk: 680kB

Sort  (cost=2796544.62..2796547.50 rows=1152 width=278) (actual time=47865.883..47868.570 rows=2421 loops=1)
  Sort Key: gtab07.patent, gtab04.product
  Sort Method: external merge  Disk: 680kB