Why Is Query Against XMLTYPE Table ACME_CUST Doing A Full Table Scan?

Question

On our Oracle Database 11g Enterprise Edition Release 11.2.0.1.0, we have a query against against a 25,214 record XMLTYPE ACME_CUST table.
SELECT rownum   AS seq,
      EID  AS eid,
      SUBSTR(CUST_ID, 1, INSTR(CUST_ID, '|')-1) AS tgt_acme_customer_id,
      SUBSTR(CUST_ID, INSTR(CUST_ID, '|')   +1) AS src_acme_customer_id_list
    FROM
      (SELECT ac.eid EID,
        listagg(ac.acme_cust_id, '|') WITHIN GROUP (
      ORDER BY ac.acme_cust_id, ac.acme_cust_id) CUST_ID
      FROM ACME_CUST ac
      GROUP BY ac.eid
      HAVING COUNT(ac.acme_cust_id)>1)Explain plan shows:
Select Statement
Count
VIEW
FILTER
Filter Predicates
COUNT(*) > 1
SORT GROUP BY
TABLE ACCESS ACME_CUST FULL
The ACME_CUST Table has a virtual column defined on acme_cust_id along with a corresponding index. This filed is also defined as a primary key.
Here is the table definitiion and associated statements:
CREATE
  TABLE "N98991"."ACME_CUST" OF XMLTYPE
    CONSTRAINT "ACME_CUST_ID_PK" PRIMARY KEY ("ACME_CUST_ID") USING INDEX
    PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS STORAGE(INITIAL 65536
    NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCREASE 0 FREELISTS 1
    FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE
    DEFAULT) TABLESPACE "ACME_DEV" ENABLE
  XMLTYPE STORE AS SECUREFILE BINARY XML
    TABLESPACE "ACME_DEV" ENABLE STORAGE IN ROW CHUNK 8192 CACHE READS LOGGING
    NOCOMPRESS KEEP_DUPLICATES STORAGE(INITIAL 106496 NEXT 1048576 MINEXTENTS 1
    MAXEXTENTS 2147483645 PCTINCREASE 0 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT
    CELL_FLASH_CACHE DEFAULT)
  ALLOW NONSCHEMA ALLOW ANYSCHEMA VIRTUAL COLUMNS
    "EID" AS (CAST(SYS_XQ_UPKXML2SQL(SYS_XQEXVAL(XMLQUERY(
    'declare default element namespace "http://www.cigna.com/acme/domains/customer/customerprofile/2011/11"; (::)                              
/customerProfile/@eid'
    PASSING BY VALUE SYS_MAKEXML(128,"XMLDATA") RETURNING CONTENT ),0,0,
    16777216,0),50,1,2) AS VARCHAR2(15))),
  *bold*  "ACME_CUST_ID" AS (CAST(SYS_XQ_UPKXML2SQL(SYS_XQEXVAL(XMLQUERY(
    'declare default element namespace "http://www.cigna.com/acme/domains/customer/customerprofile/2011/11"; (::)                              
/customerProfile/@id' *bold*
    PASSING BY VALUE SYS_MAKEXML(128,"XMLDATA") RETURNING CONTENT ),0,0,
    16777216,0),50,1,2) AS VARCHAR2(50))),
    "CRET_DT" AS (SYS_EXTRACT_UTC(CAST(TO_TIMESTAMP_TZ(SYS_XQ_UPKXML2SQL(
    SYS_XQEXVAL(XMLQUERY(
    'declare default element namespace "http://www.cigna.com/acme/domains/customer/customerprofile/2011/11"; (::)                                                                                                      
/customerProfile/@create_dt'
    PASSING BY VALUE SYS_MAKEXML(128,"XMLDATA") RETURNING CONTENT ),0,0,
    16777216,0),50,1,2),'SYYYY-MM-DD"T"HH24:MI:SS.FFTZH:TZM') AS TIMESTAMP
WITH
  TIME ZONE)))
  PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING STORAGE
    INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCREASE 0
    FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT
    CELL_FLASH_CACHE DEFAULT
  TABLESPACE "ACME_DEV" ;
CREATE
  INDEX "N98991"."ACME_CST_CRET_DT_IDX" ON "N98991"."ACME_CUST"
    "CRET_DT"
  PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS STORAGE
    INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCREASE 0
    FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT
    CELL_FLASH_CACHE DEFAULT
  TABLESPACE "ACME_DEV" ;
CREATE
  INDEX "N98991"."ACME_CST_EID_IDX" ON "N98991"."ACME_CUST"
    "EID"
  PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS STORAGE
    INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCREASE 0
    FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT
    CELL_FLASH_CACHE DEFAULT
  TABLESPACE "ACME_DEV" ;
*bold*CREATE UNIQUE INDEX "N98991"."ACME_CUST_ID_PK" ON "N98991"."ACME_CUST"
    "ACME_CUST_ID"
  PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS STORAGE *bold*
    INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCREASE 0
    FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT
    CELL_FLASH_CACHE DEFAULT
  TABLESPACE "ACME_DEV" ;
  CREATE
    INDEX "N98991"."ACME_CUST_XMLINDEX_IX" ON "N98991"."ACME_CUST"
      OBJECT_VALUE
    INDEXTYPE IS "XDB"."XMLINDEX" PARAMETERS
      'XMLTABLE ACME_CUST_IDX_TAB XMLNamespaces (''http://www.cigna.com/acme/domains/commoncontact/2011/11'' as "cm",  default ''http://www.cigna.com/acme/domains/customer/customerprofile/2011/11''),      
''/customerProfile''       
columns      
DOB date  PATH ''personInformation/cm:birthDate'',      
FIRSTNAME varchar2(40)    PATH ''name/cm:givenName'',      
LASTNAME varchar2(40)    PATH ''name/cm:surName'',      
SSN varchar2(30)    PATH ''identifiers/ssn'',      
MEMBERINFOS XMLType path ''memberInfos/memberInfo'' VIRTUAL       
XMLTable acme_cust_lev2_idx_tab XMLNAMESPACES(default ''http://www.cigna.com/acme/domains/customer/customerprofile/2011/11''),      
''/memberInfo'' passing MEMBERINFOS         
columns         
ami varchar2(40) PATH ''ami'',        
subscId varchar2(50) PATH ''clientRelationship/subscriberInformation/subscriberId'',        
employeeId varchar2(50) PATH ''systemKeys/employeeId'',        
clientId varchar2(50) PATH ''clientRelationship/clientId''      
CREATE UNIQUE INDEX "N98991"."SYS_C00384339" ON "N98991"."ACME_CUST"
    "SYS_NC_OID$"
  PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS STORAGE
    INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCREASE 0
    FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT
    CELL_FLASH_CACHE DEFAULT
  TABLESPACE "ACME_DEV" ;
CREATE UNIQUE INDEX "N98991"."SYS_IL0000649948C00003$$" ON "N98991"."ACME_CUST"
    PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAGE(INITIAL 65536 NEXT 1048576
    MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCREASE 0 FREELISTS 1 FREELIST
    GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
    TABLESPACE "ACME_DEV" PARALLEL (DEGREE 0 INSTANCES 0) ;Why isn't the unique index ACME_CUST_ID_PK on the virtual column ACME_CUST_ID being used in the explain plan?
Any input would be much appreciated, as really stumped here.
Regards,
Rick

Answer 1

Hi Richard,
The 10053 event appears overkill for this situation. What's the big deal?
Set the event, run the query, unset the event, check the trace file, that's all.
It's not overkill if it helps you understanding what happens and why an index is of no use in this situation.
Tried the /*+ INDEX_FFS(ACME_CUST_ID_PK) */ hint in the 'nested' query.Not sure what nested query you're referring to, so if I misunderstood what you mean, just ignore the following comment.
From what you posted earlier, it looks like you're talking about this part :
listagg(ac.acme_cust_id,'|') WITHIN GROUP (
ORDER BY ac.acme_cust_id,ac.acme_cust_id) CUST_IDThat's not a nested query, it's a projection. All the main work (retrieving rows) has already been done when it comes to this part.
May just have to accept the query performance as it is...Maybe you can try something else.
See the document : Oracle XML DB : Best Practices, page 15 ex. 8 :
When there are multiple scalar values that need to be grouped or ordered, it is better to write it
with XMLTable construct that projects out all columns to be ordered or grouped as shown
below.Here's an example close to your actual requirement :
Connected to:
Oracle Database 11g Express Edition Release 11.2.0.2.0 - Production
SQL> create table xtab_cols of xmltype
  2  xmltype store as securefile binary xml;
Table created.
SQL> insert /*+ append */ into xtab_cols
  2  select xmlelement("ROW",
  3           xmlforest(
  4            TABLE_NAME, COLUMN_NAME, DATA_TYPE, DATA_TYPE_MOD, DATA_TYPE_OWNER,
  5            DATA_LENGTH, DATA_PRECISION, DATA_SCALE, NULLABLE, COLUMN_ID,
  6            DEFAULT_LENGTH, NUM_DISTINCT, LOW_VALUE, HIGH_VALUE,
  7            DENSITY, NUM_NULLS, NUM_BUCKETS, LAST_ANALYZED, SAMPLE_SIZE,
  8            CHARACTER_SET_NAME, CHAR_COL_DECL_LENGTH,
  9            GLOBAL_STATS, USER_STATS, AVG_COL_LEN, CHAR_LENGTH, CHAR_USED,
10            V80_FMT_IMAGE, DATA_UPGRADED, HISTOGRAM
11           )
12         )
13  from dba_tab_cols
14  where owner = 'SYS'
15  ;
57079 rows created.
SQL> commit;
Commit complete.
SQL> set long 1000
SQL> set pages 100
SQL> select xmlserialize(document object_value) from xtab_cols where rownum = 1;
XMLSERIALIZE(DOCUMENTOBJECT_VALUE)
<ROW>
  <TABLE_NAME>ACCESS$</TABLE_NAME>
  <COLUMN_NAME>D_OBJ#</COLUMN_NAME>
  <DATA_TYPE>NUMBER</DATA_TYPE>
  <DATA_LENGTH>22</DATA_LENGTH>
  <NULLABLE>N</NULLABLE>
  <COLUMN_ID>1</COLUMN_ID>
  <NUM_DISTINCT>7454</NUM_DISTINCT>
  <LOW_VALUE>C2083A</LOW_VALUE>
  <HIGH_VALUE>C3031D18</HIGH_VALUE>
  <DENSITY>,000134156157767642</DENSITY>
  <NUM_NULLS>0</NUM_NULLS>
  <NUM_BUCKETS>1</NUM_BUCKETS>
  <LAST_ANALYZED>2012-01-28</LAST_ANALYZED>
  <SAMPLE_SIZE>34794</SAMPLE_SIZE>
  <GLOBAL_STATS>YES</GLOBAL_STATS>
  <USER_STATS>NO</USER_STATS>
  <AVG_COL_LEN>5</AVG_COL_LEN>
  <CHAR_LENGTH>0</CHAR_LENGTH>
  <V80_FMT_IMAGE>NO</V80_FMT_IMAGE>
  <DATA_UPGRADED>YES</DATA_UPGRADED>
  <HISTOGRAM>NONE</HISTOGRAM>
</ROW>
SQL> exec dbms_stats.gather_table_stats(user, 'XTAB_COLS');
PL/SQL procedure successfully completed.
SQL> set autotrace traceonly
SQL> set timing on
SQL> set lines 120
SQL> select x.table_name
  2       , listagg(x.column_name, ',') within group (order by column_id)
  3  from xtab_cols t
  4     , xmltable('/ROW' passing t.object_value
  5        columns table_name  varchar2(30) path 'TABLE_NAME'
  6              , column_name varchar2(30) path 'COLUMN_NAME'
  7              , column_id   number       path 'COLUMN_ID'
  8       ) x
  9  group by x.table_name
10  ;
4714 rows selected.
Elapsed: 00:00:08.25
Execution Plan
Plan hash value: 602782846
| Id  | Operation           | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT    |           |   466M|   101G|  1580K  (3)| 05:16:04 |
|   1 |  SORT GROUP BY      |           |   466M|   101G|  1580K  (3)| 05:16:04 |
|   2 |   NESTED LOOPS      |           |   466M|   101G|  1552K  (1)| 05:10:32 |
|   3 |    TABLE ACCESS FULL| XTAB_COLS | 57079 |    12M|   408   (1)| 00:00:05 |
|   4 |    XPATH EVALUATION |           |       |       |            |          |
Statistics
          9  recursive calls
          1  db block gets
       1713  consistent gets
          0  physical reads
         96  redo size
     773516  bytes sent via SQL*Net to client
       3873  bytes received via SQL*Net from client
        316  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
       4714  rows processedAnd of course, even better after adding a structured XML index (4714 rows fetched in 1s) :
SQL> CREATE INDEX xtab_cols_sxi ON xtab_cols (OBJECT_VALUE) INDEXTYPE IS XDB.XMLIndex
  2  PARAMETERS (
  3  q'#XMLTable my_xtab
  4  '/ROW'
  5  columns table_name varchar2(30) path 'TABLE_NAME'
  6        , column_name varchar2(30) path 'COLUMN_NAME'
  7        , column_id number path 'COLUMN_ID' #');
Index created.
Elapsed: 00:00:13.42
SQL> select x.table_name
  2       , listagg(x.column_name, ',') within group (order by column_id)
  3  from xtab_cols t
  4     , xmltable('/ROW' passing t.object_value
  5        columns table_name  varchar2(30) path 'TABLE_NAME'
  6              , column_name varchar2(30) path 'COLUMN_NAME'
  7              , column_id   number       path 'COLUMN_ID'
  8       ) x
  9  group by x.table_name
10  ;
4714 rows selected.
Elapsed: 00:00:01.00
Execution Plan
Plan hash value: 3303494605
| Id  | Operation          | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |         | 57520 |  3201K|   174   (3)| 00:00:03 |
|   1 |  SORT GROUP BY     |         | 57520 |  3201K|   174   (3)| 00:00:03 |
|   2 |   TABLE ACCESS FULL| MY_XTAB | 57520 |  3201K|   171   (1)| 00:00:03 |
Note
   - dynamic sampling used for this statement (level=2)
Statistics
        297  recursive calls
          1  db block gets
        989  consistent gets
          0  physical reads
        176  redo size
     773516  bytes sent via SQL*Net to client
       3873  bytes received via SQL*Net from client
        316  SQL*Net roundtrips to/from client
         21  sorts (memory)
          0  sorts (disk)
       4714  rows processed