20 Index Restriction on Remote Tables (i.e. using Database Links)

Question

The Oracle Database Administrator's Guides for 10g and 11g document a performance restriction that "No more than 20 indexes are considered for a remote table." If I go back to the 8i documentation it says "In cost-based optimization, no more than 20 indexes per remote table are considered when generating query plans. The order of the indexes varies; if the 20-index limitation is exceeded, random variation in query plans may result."
Does anyone have more details on this performance restriction? In particular I am trying to answer these questions:
1) Are the 20 indexes which are considered by the CBO still random in 10g?
2) Can I influence which indexes are considered with index hints or will my hints only be considered if they are for one of the "random" 20 indexes which are being considered by the CBO?
3) Are there any other approaches or work-arounds to this restriction assuming you need to select from a large remote table with more than 20 indexes (and need to perform the selection using 1 of those indexes to get adequate performance) or do we need to abandon database links for this table?
Thanks in advance for your input.

Answer 1

So, here's my simple test.
SQL>
SQL> create table gurnish.indexes20plus ( n1 number, n2 number, n3 number, n4 number, n5 number, n6 number, n7 number,
2 n8 number, n9 number, n10 number, n11 number, n12 number, n13 number, n14 number, n15 number, n16 number,
3 n17 number, n18 number, n19 number, n20 number, n21 number, n22 number, n23 number, n24 number,
4 n25 number, n26 number, n28 number);
create index xin1 on indexes20plus (n1);
Table created.
SQL> SQL> create index xin2 on indexes20plus (n2);
create index xin3 on indexes20plus (n3);
Index created.
SQL> SQL>
Index created.
SQL> SQL> create index xin4 on indexes20plus (n4);
Index created.
SQL> SQL>
Index created.
SQL> SQL> create index xin5 on indexes20plus (n5);
create index xin6 on indexes20plus (n6);
Index created.
SQL> SQL>
Index created.
SQL> SQL> create index xin7 on indexes20plus (n7);
Index created.
SQL> SQL> create index xin8 on indexes20plus (n8);
Index created.
SQL> SQL> create index xin9 on indexes20plus (n9);
Index created.
SQL>
SQL> create index xin10 on indexes20plus (n10);
Index created.
SQL> SQL> create index xin11 on indexes20plus (n11);
create index xin12 on indexes20plus (n12);
create index xin13 on indexes20plus (n13);
Index created.
SQL> SQL>
Index created.
SQL> SQL>
Index created.
SQL> SQL> create index xin14 on indexes20plus (n14);
Index created.
SQL> SQL> create index xin15 on indexes20plus (n15);
Index created.
SQL>
SQL> create index xin16 on indexes20plus (n16);
Index created.
SQL>
SQL> create index xin17 on indexes20plus (n17);
Index created.
SQL> SQL> create index xin18 on indexes20plus (n18);
Index created.
SQL> SQL> create index xin19 on indexes20plus (n19);
Index created.
SQL> SQL> create index xin20 on indexes20plus (n20);
Index created.
SQL> SQL> create index xin21 on indexes20plus (n21);
Index created.
declare
i number;
begin
for i in 1..100
loop
dbms_random.seed(i+100);
insert into indexes20plus values (dbms_random.value(1,5),dbms_random.value(1,21),dbms_random.RANDOM, dbms_random.RANDOM,dbms_random.value(1,20),
dbms_random.value(1,4),dbms_random.value(1,6), dbms_random.value(1,7),dbms_random.value(1,9),dbms_random.value(1,10),
dbms_random.value(1,11),dbms_random.value(1,12),dbms_random.value(1,13),dbms_random.value(1,14),dbms_random.value(1,1),
dbms_random.value(1,1),dbms_random.value(1,19),dbms_random.value(1,122),dbms_random.value(1,20),dbms_random.value(1,20)
,dbms_random.value(4,20),dbms_random.value(1,20),dbms_random.value(1,20),dbms_random.value(1,20),dbms_random.value(1,20)
,dbms_random.value(4,20),dbms_random.value(4,20));
end loop;
commit;
end;
SQL> set autotrace traceonly
SQL> l
1* select * from gurnish.indexes20plus@lvoprds where n1 = 4
SQL> /
no rows selected
Execution Plan
Plan hash value: 441368878
| Id | Operation | Name | Rows | Bytes | Cost (%CPU
)| Time | Inst |
| 0 | SELECT STATEMENT REMOTE | | 1 | 351 | 1 (0
)| 00:00:01 | |
| 1 | TABLE ACCESS BY INDEX ROWID| INDEXES20PLUS | 1 | 351 | 1 (0
)| 00:00:01 | LVPRD |
|* 2 | INDEX RANGE SCAN | XIN1 | 1 | | 1 (0
)| 00:00:01 | LVPRD |
Predicate Information (identified by operation id):
2 - access("A1"."N1"=4)
Note
- fully remote statement
- dynamic sampling used for this statement
Statistics
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
1897 bytes sent via SQL*Net to client
481 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
SQL> select * from gurnish.indexes20plus@lvoprds where n21 = 4;
no rows selected
Execution Plan
Plan hash value: 2929530649
| Id | Operation | Name | Rows | Bytes | Cost (%CPU
)| Time | Inst |
| 0 | SELECT STATEMENT REMOTE | | 1 | 351 | 1 (0
)| 00:00:01 | |
| 1 | TABLE ACCESS BY INDEX ROWID| INDEXES20PLUS | 1 | 351 | 1 (0
)| 00:00:01 | LVPRD |
|* 2 | INDEX RANGE SCAN | XIN21 | 1 | | 1 (0
)| 00:00:01 | LVPRD |
Predicate Information (identified by operation id):
2 - access("A1"."N21"=4)
Note
- fully remote statement
- dynamic sampling used for this statement
Statistics
1 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
1897 bytes sent via SQL*Net to client
481 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
SQL>