lightdb22.2-增强对oracle hint的兼容

Posted 2022-12-08 紫无之紫

lightdb22.2-增强对 oracle 常用 hint 的兼容(一)

表访问方式 (table access)

• 支持 oracle 的full建议优化器对表进行全表扫描获取数据, 与 lightdb 原先的 seqscan hint相同
• 支持 oracle 的index建议优化器对表进通过索引获取数据，与 lightdb 原先的 indexscan hint 相同

表关联方法(join method)

lightdb 支持3种表的join算法，分别为：hashjoin、nestloop以及mergejoin。
原先支持使用hashjoin, nestloop, mergejoin 来建议优化器对指定表的join使用特定算法。

现支持 oracle 的use_hash, use_nl,use_merge, no_use_hash, no_use_nl, no_use_merge 来建议优化器对以指定的表作为内表的 join 使用特定算法。

要使用上述 oracle 语义需要设置lightdb_syntax_compatible_type：

• 当lightdb_syntax_compatible_type = 'oracle'，use_xxx 与 hashjoin 等 hint都是用来建议优化器对以指定的表作为内表的 join 使用特定算法，此时可以指定单表，当指定多个表时，效果相当于对每个表都使用use_xxx，比如：/*+use_hash(a b c)*/ 即是 /*+use_hash(a) use_hash(b) use_hash(c)*/
• 当 lightdb_syntax_compatible_type = 'off'时，use_xxx 与 hashjoin 等 hint 都是用来建议优化器对指定表的join使用特定算法，此时不能指定单表。

从下面的例子可以看出这两类 hint 的区别:

lightdb@postgres=# set lightdb_syntax_compatible_type = 'off';
lightdb@postgres=# EXPLAIN (COSTS false) SELECT /*+leading(t1 t2 t3) use_nl(t1 t2) use_merge(t1 t2 t3)*/* FROM t1, t2 ,t3 WHERE t1.id = t2.id and t2.id=t3.id;
LOG:  pg_hint_plan:
used hint:
USE_NL(t1@lt#0 t2@lt#0)
USE_MERGE(t1@lt#0 t2@lt#0 t3@lt#0)
Leading(((t1@lt#0 t2@lt#0) t3@lt#0))
not used hint:
duplication hint:
error hint:

                    QUERY PLAN                     
----------------------------------------------------
Merge Join
Merge Cond: (t1.id = t3.id)
->  Nested Loop
        ->  Index Scan using t1_pkey on t1 @"lt#0"
        ->  Index Scan using t2_pkey on t2 @"lt#0"
            Index Cond: (id = t1.id)
->  Index Scan using t3_pkey on t3 @"lt#0"
(7 rows)

lightdb@postgres=# 

lightdb@postgres=# set lightdb_syntax_compatible_type = 'oracle';
lightdb@postgres=# EXPLAIN (COSTS false) SELECT /*+leading(t1 t2 t3) use_nl(t2) use_merge(t3)*/* FROM t1, t2 ,t3 WHERE t1.id = t2.id and t2.id=t3.id;
LOG:  pg_hint_plan:
used hint:
USE_NL(t2@lt#0)
USE_MERGE(t3@lt#0)
Leading(((t1@lt#0 t2@lt#0) t3@lt#0))
not used hint:
duplication hint:
error hint:

                    QUERY PLAN                     
----------------------------------------------------
Merge Join
Merge Cond: (t1.id = t3.id)
->  Nested Loop
        ->  Index Scan using t1_pkey on t1 @"lt#0"
        ->  Index Scan using t2_pkey on t2 @"lt#0"
            Index Cond: (id = t1.id)
->  Index Scan using t3_pkey on t3 @"lt#0"
(7 rows)

表关联顺序(join order)

lightdb 原先支持使用 leading 来指定 join 顺序，至少需要指定两张表，如：leading(a b)。

对 oracle 的兼容有以下两项

支持 oracle 的 ordered

ordered hint 用来强制使表按 from 后的表顺序去 join。

lightdb@postgres=# EXPLAIN  SELECT/*+ordered*/ * FROM t3, t2, t1 WHERE t1.id = t2.id;
LOG:  pg_hint_plan:
used hint:
Leading(@lt#0 ((t3@lt#0 t2@lt#0) t1@lt#0))
not used hint:
duplication hint:
error hint:

                                QUERY PLAN                                   
--------------------------------------------------------------------------------
Hash Join  (cost=60.85..77409.29 rows=5107600 width=24)
Hash Cond: (t2.id = t1.id)
->  Nested Loop  (cost=0.00..63915.85 rows=5107600 width=16)
        ->  Seq Scan on t3 @"lt#0"  (cost=0.00..32.60 rows=2260 width=8)
        ->  Materialize  (cost=0.00..43.90 rows=2260 width=8)
            ->  Seq Scan on t2 @"lt#0"  (cost=0.00..32.60 rows=2260 width=8)
->  Hash  (cost=32.60..32.60 rows=2260 width=8)
        ->  Seq Scan on t1 @"lt#0"  (cost=0.00..32.60 rows=2260 width=8)
(8 rows)

支持单表的leading

leading 用来强制指定表在 join 时作为外表去 join，如: leading(a)。

lightdb@postgres=# EXPLAIN  SELECT/*+leading(t3)*/ * FROM t3, t2, t1 WHERE t1.id = t2.id;
LOG:  pg_hint_plan:
used hint:
Leading(t3@lt#0)
not used hint:
duplication hint:
error hint:

                                        QUERY PLAN                                        

-------------------------------------------------------------------------------------------
-
Nested Loop  (cost=10000000000.16..10000064316.68 rows=5107600 width=24)
->  Seq Scan on t3 @"lt#0"  (cost=0.00..32.60 rows=2260 width=8)
->  Materialize  (cost=10000000000.16..10000000444.73 rows=2260 width=16)
        ->  Nested Loop  (cost=10000000000.16..10000000433.43 rows=2260 width=16)
            ->  Seq Scan on t2 @"lt#0"  (cost=0.00..32.60 rows=2260 width=8)
            ->  Index Scan using t1_pkey on t1 @"lt#0"  (cost=0.15..0.18 rows=1 width=8)
                    Index Cond: (id = t2.id)
(7 rows)

lightdb@postgres=# 

lt#0" (cost=0.15…0.18 rows=1 width=8)
Index Cond: (id = t2.id)
(7 rows)

lightdb@postgres=#
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