Oracle在建立索引時,不會儲存NULL值,而PostgreSQL在建立索引時則會儲存NULL值.在查詢時,如使用Column is null這樣的條件查詢,Oracle不會使用索引而PostgreSQL則會使用索引.

**Oracle**
插入資料,200w多行的資料,然後插入一行值為null的資料.
```
TEST-orcl@DESKTOP-V430TU3>create table tbl1(id int);

Table created.
TEST-orcl@DESKTOP-V430TU3>create global temporary table tmp(id int);

Table created.
TEST-orcl@DESKTOP-V430TU3>insert into tmp select rownum from dba_objects;

133456 rows created.
TEST-orcl@DESKTOP-V430TU3>insert into tmp select * from tmp;

133455 rows created.

TEST-orcl@DESKTOP-V430TU3>/

266910 rows created.

TEST-orcl@DESKTOP-V430TU3>/

533820 rows created.

TEST-orcl@DESKTOP-V430TU3>/

1067640 rows created.
TEST-orcl@DESKTOP-V430TU3>insert into tbl1 select * from tmp;

2135296 rows created.

TEST-orcl@DESKTOP-V430TU3>commit;

Commit complete.
TEST-orcl@DESKTOP-V430TU3>exec dbms_stats.gather_table_stats('TEST','TBL1',cascade=>true);

PL/SQL procedure successfully completed.

TEST-orcl@DESKTOP-V430TU3>select index_name,index_type,blevel,leaf_blocks,num_rows,status,distinct_keys from user_indexes  where table_name='TBL1';

INDEX_NAME                     INDEX_TYPE                      BLEVEL
------------------------------ --------------------------- ----------
LEAF_BLOCKS   NUM_ROWS STATUS   DISTINCT_KEYS
----------- ---------- -------- -------------
IDX_TBL1_ID                    NORMAL                               2
       4662    2103843 VALID           134688
TEST-orcl@DESKTOP-V430TU3>insert into tbl1 values(null);

1 row created.

TEST-orcl@DESKTOP-V430TU3>commit;

Commit complete.

TEST-orcl@DESKTOP-V430TU3>
TEST-orcl@DESKTOP-V430TU3>exec dbms_stats.gather_table_stats('TEST','TBL1',cascade=>true);

PL/SQL procedure successfully completed.

TEST-orcl@DESKTOP-V430TU3>select index_name,index_type,blevel,leaf_blocks,num_rows,status,distinct_keys from user_indexes  where table_name='TBL1';

INDEX_NAME                     INDEX_TYPE                      BLEVEL
------------------------------ --------------------------- ----------
LEAF_BLOCKS   NUM_ROWS STATUS   DISTINCT_KEYS
----------- ---------- -------- -------------
IDX_TBL1_ID                    NORMAL                               2
       4771    2152683 VALID           134688

```
執行查詢
```
TEST-orcl@DESKTOP-V430TU3>set autotrace on explain
TEST-orcl@DESKTOP-V430TU3>select * from tbl1 where id is null;

ID
----------

Execution Plan
----------------------------------------------------------
Plan hash value: 312383637

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |     5 |   898   (2)| 00:00:11 |
|*  1 |  TABLE ACCESS FULL| TBL1 |     1 |     5 |   898   (2)| 00:00:11 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("ID" IS NULL)

TEST-orcl@DESKTOP-V430TU3>

```

**PostgreSQL**
資料表tbl1結構與Oracle一致.
```
testdb=# insert into tbl1 select generate_series(1,100000);
INSERT 0 100000
testdb=# explain (analyze,verbose)  select * from tbl1 where id is null;
                                                 QUERY PLAN                                                 
--------------------------------------------------------------------------------------------------------
 Seq Scan on public.tbl1  (cost=0.00..1569.33 rows=11 width=4) (actual time=26.052..130.752 rows=1 loops=1)
   Output: id
   Filter: (tbl1.id IS NULL)
   Rows Removed by Filter: 110000
 Planning Time: 1.403 ms
 Execution Time: 130.814 ms
(6 rows)

testdb=# create index idx_tb1_id on tbl1(id);
CREATE INDEX
testdb=# explain (analyze,verbose)  select * from tbl1 where id is null;
                                                         QUERY PLAN                                                          
--------------------------------------------------------------------------------------------------------
 Index Only Scan using idx_tb1_id on public.tbl1  (cost=0.42..8.56 rows=4 width=4) (actual time=0.133..0.136 rows=1 loops=1)
   Output: id
   Index Cond: (tbl1.id IS NULL)
   Heap Fetches: 1
 Planning Time: 1.512 ms
 Execution Time: 0.199 ms
(6 rows)
```
使用id is null進行查詢,使用的是Index Only Scan.

Oracle vs PostgreSQL，研發注意事項（12） - NULL與索引

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