DELETE FROM test WHERE object_id IS NULL;
COMMIT;
ANALYZE TABLE test compute STATISTICS FOR TABLE FOR ALL indexes FOR ALL indexed COLUMNS;
EXPLAIN PLAN FOR SELECT Count(*) FROM test;
SELECT * FROM TABLE(dbms_xplan.display);
Plan hash value: 1950795681
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| Id | Operation | Name | Rows | Cost (%CPU)| Time |
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| 0 | SELECT STATEMENT | | 1 | 135 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| TEST | 41790 | 135 (2)| 00:00:02 |
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综上，看起来，假如索引列的表结构非空，则会走索引；若索引列表结构可空，则无论是否存在null资料，都会走全表；可以用where 索引列 is not null过滤空资料，则还是会走索引。