SQL implicit conversion brought about by performance issues

Copyright attribution: Tiankai Technology

This link: https://blog.csdn.net/dbs_service/article/details/102760899

Hit a slow SQL exception from the implementation plan view, and take the normal index, after further analysis:
the original SQL statement:

SELECT ROWNUM AS ROWNUMBER__,T.*
  FROM (Select T1.ID          AS "InvMainID",
               T2.CONSIGNCODE AS "ConsignCode",               
               T2.ORDERCODE  AS "OrderCode",
               T3.CREATETIME AS "CreateTime",
               T2.CUSCODE    AS "CusCode",
               T2.BILLDATE   AS "OutTime",
               T4.ISCANCEL   AS "IsCancel",
               T1.ISPRINT    AS "IsPrint",
               T3.TITLE      AS "Title",
               T3.ACCEPTER   AS "Accepter",
               T3.POSTCODE   AS "PostCode",
               T2.INVAMT     AS  "InvAmt",
               T3.INVCONTENT AS "InvContent",
               T1.PRINTOR    AS "HandleMan",
               T4.ISRED      AS "IsRed",
               T4.INVNO      AS "InvNo",
               T4.INVCODE    AS "InvCode",
               T1.PRINTTIME  AS "PrintTime",
               T2.INVAMT     AS "ConsignAmt",
               T3.ADDRESS    AS "Address",
               T2.ORGCODE    AS "OrgCode"        
          From mbs7_inv.INV_CONSIGN T2        
          Join mbs7_inv.INV_INVORDER T3
            on T2.ORDERCODE = T3.ORDERCODE        
          Join mbs7_inv.WL_PSINFO T5
            on T2.CONSIGNCODE = T5.CONSIGNCODE        
          left Join mbs7_inv.Inv_InvMain T1
            on T2.CONSIGNCODE = T1.CONSIGNCODE        
          left Join mbs7_inv.Inv_Record T4
            on T1.ID = T4.InvMainID        
         Where ((((T2.INVAMT >= 1) And (T2.ISCANCEL = 0)) And
               (T3.STATUS = 0)) And (T5.PSRESULT = 1))
         ORDER BY T4.INVNO ASC, T1.ISPRINT DESC) T 
 WHERE T4."InvCode" = '144011320565'
   AND T4."InvNo" >= '00011109'
   AND T4."InvNo" <= '00011163'
   AND ROWNUM   <= 100

Implementation plan are as follows:

Here is the main part to achieve a red implicit conversion connection, and cause abnormal execution plan appears, COST increments great phenomenon:
the first performance is very slow, 1-2 minutes before the results came out, the application time-outs, and after performing a second faster, it can be 5 seconds.

Analysis:
try to create an index to accept sql_profile way optimized, but the effect is not obvious, the first execution of still very slow;
later found the execution plan for the statement consume a larger portion of the cost of such implicit conversion exists SYS_OP_C2C comparison, then
found table T1 and T2 are connected to the field and NVARCHAR2 varchar2, since the circulation through an implicit conversion, in this case
the final COST large, and the second performance, since the converted value already exists in the SGA, i.e., so a direct comparison available, the rate
quickly, in order to prove this point, after then refresh the SGA, to reproduce the problem immediately.

Solution:
Create an index, and the table T1 connection field into varchar2, implicit conversion implementation plan disappeared, COST significant decline in the first
execution within 3 seconds, the second time within 2 seconds after the execution, cost decreased from 100 to over 20,000.

Embodiment Operation:

SQL> CREATE INDEX mbs7_inv.ix_code_PSRESULT on mbs7_inv.wl_psinfo(CONSIGNCODE,PSRESULT) tablespace inv_index;
SQL> drop index mbs7_inv.IX_WL_PSINFO_PSRESULT;
SQL> alter table mbs7_inv.INV_INVMAIN modify CONSIGNCODE VARCHAR2(20);
 
Table altered

The optimized execution plan:

After eliminating the implicit conversion here, plan to use the normal index connection, COST significant decline!

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