Improving performance: Very Slow Oracle SQL Join

Question

I am a newbie in SQL querying and I am spending 3hrs to get the whole result of joining 2 queries. I have focused on using left joins and avoided using subqueries on the select statement after researching. However it is still extremely slow. I have no close friends who know sql enough to explain whats wrong or what I approach I should take. I am also new here so if this question is not allowed please inform me and I will remove it immediately.

This is the structure of the query... The first query will get the member details. The second query will get the transaction details. The relationship is, one product has many sub-plans which has many members. One product also has many transactions which is made on a per product basis. I am required to show all transactions and duplicate each line for each member. I joined the queries using the product primary key. Prior to joining, I have tested both individual queries and they turned out fine. Only 1-2 secs and I get the result. But joining the two, I end up with 3 hrs of waiting.

SELECT
MPPFF.N_DX,
MPPFF.PM_A_P,
MPPFF.FEE1,
MPPFF.FEE2,
MPPFF.FEE3,
MPPFF.FEE4,
MPPFF.FEE11,
MPPFF.FEE12,
MPPFF.FEE5,
MPPFF.N_NO,
MPPFF.SETN_DX,
MPPFF.PRIME_NO,
MPPFF.SECN_NO,
MPPFF.COMM_A,
MPPFF.TYX_NO,
MPPFF.P_NAME,
MPPFF.B_BFX,
MPPFF.B_FM,
MPPFF.B_TO,
MPPFF.BB_NAME_P,
MPPFF.BB_NAME_S,
MPPFF.REVERSE_BFX,
MPPFF.TYX_REF_NO,
MPPFF.BB_NO_AX,
MPPFF.BB_NAME_AX,
MPPFF.DXC,
MPPFF.ST,
MPPFF.DAY,
MPPFF.CE_D_PRODUCT,
MPPFF.CE_H,
MPPFF.AS_C_E,
MPPFF.BCH,
MPPFF.RCPY_NO,
MPPFF.RE_BFX,
MPPFF.A_END,
MPPFF.PLACE,
MPPFF.MEMB_DX,
MPPFF.MBR_NO,
MPPFF.MBR_TR_BFX,
MPPFF.CE_D_TERM_CE,
MPPFF.MEMBER_AS,
MPPFF.C_USER,
MPPFF.C_BFX,
MPPFF.U_USER,
MPPFF.U_BFX

FROM (
        SELECT
        FF.N_DX,
        FF.PM_A_P,
        FF.FEE1,
        FF.FEE2,
        FF.FEE3,
        FF.FEE4,
        FF.FEE11,
        FF.FEE12,
        FF.FEE5,
        FF.N_NO,
        FF.SETN_DX,
        FF.PRIME_NO,
        FF.SECN_NO,
        FF.COMM_A,
        FF.TYX_NO,
        FF.P_NAME,
        FF.B_BFX,
        FF.B_FM,
        FF.B_TO,
        FF.BB_NAME_P,
        FF.BB_NAME_S,
        FF.REVERSE_BFX,
        FF.TYX_REF_NO,
        FF.BB_NO_AX,
        FF.BB_NAME_AX,
        FF.DXC,
        FF.ST,
        FF.DAY,
        FF.CE_D_PRODUCT,
        FF.CE_H,
        FF.AS_C_E,
        FF.RCPY_NO,
        FF.RE_BFX,
        FF.A_END,
        FF.BCH,
        MPP.MBR_NO,
        MPP.MBR_TR_BFX,
        MPP.CE_D_TERM_CE,
        MPP.C_USER,
        MPP.C_BFX,
        MPP.U_USER,
        MPP.U_BFX,
        MPP.PLACE,
        MPP.MEMBER_AS,
        MPP.TYX_DX,
        MPP.AS_DX,
        MPP.PRODUCT,
        MPP.POPL_DX,
        MPP.MEMB_DX,
        FF.TYX_DX

        FROM (
                SELECT
                MBR.MEMB_DX,
                MBR.MBR_NO,
                MBR.MBR_TR_BFX,
                MBR.CE_D_TERM_CE,
                MBR.C_USER,
                MBR.C_BFX,
                MBR.U_USER,
                MBR.U_BFX,
                MPP.PLACE,
                MPP.MEMBER_AS,
                MPP.TYX_DX,
                MPP.AS_DX,
                MPP.PRODUCT,
                MPP.POPL_DX

                FROM (
                        SELECT
                        MPP.PLACE,
                        MPP.MEMBER_AS,
                        MPP.TYX_DX,
                        MPP.AS_DX,
                        MPP.PRODUCT,
                        MPP.POPL_DX,
                        MMP.MEMB_DX

                        FROM(
                                SELECT
                                MPP.PLACE,
                                MPP.TYX_AS_DXC MEMBER_AS,
                                MPP.TYX_DX,
                                MPP.AS_DX,
                                MPP.POPL_DX,
                                RPT.PRODUCT

                                FROM
                                TABLE1 MPP

                                LEFT JOIN (
                                        SELECT
                                        SUBSTR(CE_D_PRODUCT,9) PRODUCT,
                                        AS_DX
                                        FROM
                                        TABLE6 RPT,
                                        TABLE7 PP
                                        WHERE
                                        PP.PRTY_DX = RPT.PRTY_DX
                                ) RPT
                                ON  MPP.AS_DX = RPT.AS_DX

                        ) MPP

                        LEFT JOIN (
                                SELECT
                                POPL_DX,
                                MEMB_DX
                                FROM
                                TABLE4
                        )MMP
                        ON MPP.POPL_DX=MMP.POPL_DX

                ) MPP,
                (
                        SELECT
                        MBR.MEMB_DX,
                        MBR.MBR_NO,
                        MBR.TERM_BFX MBR_TR_BFX,
                        MBR.CE_D_TERM_CE,
                        MBR.C_USER,
                        MBR.C_BFX,
                        MBR.U_USER,
                        MBR.U_BFX

                        FROM
                        TABLE8 MBR
                ) MBR
                WHERE
                MPP.MEMB_DX = MBR.MEMB_DX
        ) MPP
        INNER JOIN
        (
                SELECT
                FF.N_DX,
                ROUND(CB.FEE5 * FF.RATE,2) PM_A_P,
                CB.FEE1,
                CB.FEE2,
                CB.FEE3,
                CB.FEE4,
                CB.FEE11,
                CB.FEE12,
                CB.FEE5,
                FF.N_NO,
                FF.SETN_DX,
                FF.PRIME_NO,
                FF.SECN_NO,
                FF.COMM_A,
                FF.TYX_NO,
                FF.P_NAME_1||', '||FF.P_NAME_2||' '||FF.P_NAME_3 P_NAME,
                FF.B_BFX,
                FF.B_FM,
                FF.B_TO,
                FF.BB_NAME_1_P||', '||FF.BB_NAME_2_P BB_NAME_P,
                FF.BB_NAME_1_S||', '||FF.BB_NAME_2_S BB_NAME_S,
                CB.REVERSE_BFX,
                FF.TYX_REF_NO,
                FF.BB_NO_AX,
                FF.BB_NAME_1_AX||' '|| FF.BB_NAME_2_AX BB_NAME_AX,
                CASE 
                        WHEN FF.CE_D_ST IN ('A', 'B', 'C') THEN 'AC'
                        WHEN FF.DAY >1 THEN 'NEW'
                        ELSE 'AB'
                END DXC,
                FF.CE_D_ST ST,
                FF.DAY,
                FF.CE_D_PRODUCT,
                FF.CE_D_COMP CE_H,
                FF.AS_C AS_C_E,
                FF.RCPY_NO,
                FF.RE_BFX,
                ROUND(CB.A_S,2) A_END,
                FF.TYX_DX,
                MP.BCH

                FROM
                TABLE2 CB,
                TABLE3 FF

                LEFT JOIN (
                        SELECT
                        SUBSTR(CE_D_BCH_O,13) BCH,
                        TYX_DX
                        FROM
                        TABLE5 MP
                )MP
                ON MP.TYX_DX = FF.TYX_DX

                WHERE
                FF.SETN_DX = CB.SETN_DX AND
                EXTRACT( YEAR FROM FF.EFF_BFX) >=2013
        ) FF    
        ON MPP.TYX_DX = FF.TYX_DX

)MPPFF
;

Answer 1

Use ROWNUM to prevent optimizer transformations from degrading the performance.

You are encountering a common problem - two queries run fast separately but run slow when put together. Oracle does not have to run the queries in the order they are written. It can merge views, push predicates around, and generally completely re-write the query to run in a different order. Normally this is a great thing because you don't want to have to worry about which physical order to join tables. But sometimes Oracle applies the wrong transformations and the results are disastrous.

There are two ways to solve these problems.

1. Look at table structures, the statements, the execution plans, SQL monitoring or traces, statistics, etc. Try to find out which operation is slow, and why (use cardinality as your guide), and then try to fix it. This process can easily take hours, maybe even days, but it's the best way to learn.
2. Stop the optimizer from combining the queries with a simple trick. There are a few ways to do this but in my experience the simplest way is to add the pseudo-column ROWNUM to any inline view that you do not want transformed. ROWNUM is a special column that tells Oracle "this query block must be returned in a specific way, don't do anything to it".

Change this:

--This is slow:
select ...
from
(
    --This is fast:
    select ...
) inline_view1
join
(
    --This is fast:
    select ...
) inline_view2
    on ...

to this:

--Now this is fast.
select ...
from
(
    --This is fast:
    select rownum /*add rownum to prevent slow transformations*/, ...
) inline_view1
join
(
    --This is fast:
    select rownum /*add rownum to prevent slow transformations*/, ...
) inline_view2
    on ...

In your code I believe the two inline views to modify would be the outer-most MPP and FF.

---

On a side note, I disagree with with some of the other comments and answers.

• A CTE will not help here since none of the tables are used twice.
• You don't always need to know a million details about the query to tune it. Unless you have the time and want to improve your skills.
• I think your over-all query structure is good. You are on the right path to building great SQL statements. Inline views are the key to writing SQL - build small units of code, combine them in simple steps, repeat. Putting all the tables together in one massive join is a recipe for spaghetti code. Although I agree with others that you should avoid the old-fashioned join syntax. And the query would really benefit from some comments and more meaningful names. And don't feel afraid to put all the select list items on one line. Having a 500-column line isn't ideal, but you want to focus on the joins, not the simple list of columns.

Answer 2

Your query is almost unreadable, because of all the nesting. And you are mixing pre 1992 style joins with current join syntax. Don't use the outdated comma-separated join syntax. It is prone to errors. All your outer-joins are void, because at some point you will always have criteria that dismisses outer-joined records, such as when inner-joining table8 on the outer-joined table4's memb_dx.

Your query seems to translate to

select
  <several fields from the tables>
from table1 mpp
join table6 rpt on rpt.as_dx = mpp.as_dx
join table7 pp on pp.prty_dx = rpt.prty_dx
join table4 mmp on mmp.popl_dx = mpp.popl_dx
join table8 mbr on mpp.memb_dx = mmp.memb_dx
join table3 ff on ff.tyx_dx = mpp.tyx_dx and extract(year from ff.eff_bfx) >= 2013
join table2 cb on ff.setn_dx = cb.setn_dx
left join table5 mp on mp.tyx_dx = ff.tyx_dx;

and maybe you want it to be

select
  <several fields from the tables>
from table1 mpp
left join table6 rpt on rpt.as_dx = mpp.as_dx
left join table7 pp on pp.prty_dx = rpt.prty_dx
left join table4 mmp on mmp.popl_dx = mpp.popl_dx
left join table8 mbr on mpp.memb_dx = mmp.memb_dx
join table3 ff on ff.tyx_dx = mpp.tyx_dx and extract(year from ff.eff_bfx) >= 2013
join table2 cb on ff.setn_dx = cb.setn_dx
left join table5 mp on mp.tyx_dx = ff.tyx_dx;

instead or something along the lines. Get rid of all the nesting and stay with a clear and easy to read from clause.