LightDB-opt_param hint支持

LightDB 从23.4 版本开始支持opt_param hint, 用来设置优化器相关的guc参数，用法和oracle的opt_param 相同。

• 设置的参数需要是 LightDB的参数（不是oracle的参数）。
• 参数名用单引号括起来
• 字符串型的参数值也需要用单引号括起来
• 作用于整条SQL

具体用法参考如下示例：

示例

表结构

create table test_opt_param1(key1 int, key2 int, primary key(key1));
create table test_opt_param2(key1 int, key2 int, primary key(key1));
create table test_opt_param3(key1 int, key2 int, primary key(key1));

基本用法

EXPLAIN (COSTS false) select * from test_opt_param1 a join test_opt_param2 b on a.key1=b.key1;
                QUERY PLAN                 
-------------------------------------------
 Hash Join
   Hash Cond: (a.key1 = b.key1)
   ->  Seq Scan on test_opt_param1 a
   ->  Hash
         ->  Seq Scan on test_opt_param2 b
(5 rows)

EXPLAIN (COSTS false) select/*+opt_param('enable_hashjoin', 'off')*/ * from test_opt_param1 a join test_opt_param2 b on a.key1=b.key1;
LOG:  lt_hint_plan:
used hint:
opt_param('enable_hashjoin' 'off')
not used hint:
duplication hint:
error hint:

                                QUERY PLAN                                
--------------------------------------------------------------------------
 Merge Join
   Merge Cond: (a.key1 = b.key1)
   ->  Index Scan using test_opt_param1_pkey on test_opt_param1 a @"lt#0"
   ->  Index Scan using test_opt_param2_pkey on test_opt_param2 b @"lt#0"
(4 rows)

EXPLAIN (COSTS false) select/*+opt_param('enable_hashjoin', 'false')*/ * from test_opt_param1 a join test_opt_param2 b on a.key1=b.key1;
LOG:  lt_hint_plan:
used hint:
opt_param('enable_hashjoin' 'false')
not used hint:
duplication hint:
error hint:

                                QUERY PLAN                                
--------------------------------------------------------------------------
 Merge Join
   Merge Cond: (a.key1 = b.key1)
   ->  Index Scan using test_opt_param1_pkey on test_opt_param1 a @"lt#0"
   ->  Index Scan using test_opt_param2_pkey on test_opt_param2 b @"lt#0"
(4 rows)

EXPLAIN (COSTS false) select/*+opt_param('enable_hashjoin', 0)*/ * from test_opt_param1 a join test_opt_param2 b on a.key1=b.key1;
LOG:  lt_hint_plan:
used hint:
opt_param('enable_hashjoin' 0)
not used hint:
duplication hint:
error hint:

                                QUERY PLAN                                
--------------------------------------------------------------------------
 Merge Join
   Merge Cond: (a.key1 = b.key1)
   ->  Index Scan using test_opt_param1_pkey on test_opt_param1 a @"lt#0"
   ->  Index Scan using test_opt_param2_pkey on test_opt_param2 b @"lt#0"
(4 rows)

EXPLAIN (COSTS false) select/*+opt_param('enable_hashjoin', 1)*/ * from test_opt_param1 a join test_opt_param2 b on a.key1=b.key1;
LOG:  lt_hint_plan:
used hint:
opt_param('enable_hashjoin' 1)
not used hint:
duplication hint:
error hint:

                    QUERY PLAN                     
---------------------------------------------------
 Hash Join
   Hash Cond: (a.key1 = b.key1)
   ->  Seq Scan on test_opt_param1 a @"lt#0"
   ->  Hash
         ->  Seq Scan on test_opt_param2 b @"lt#0"
(5 rows)

错误用法

EXPLAIN (COSTS false) select/*+opt_param(enable_hashjoin, 'off')*/ * from test_opt_param1 a join test_opt_param2 b on a.key1=b.key1;
INFO:  lt_hint_plan: hint syntax error at or near "opt_param(enable_hashjoin, 'off')"
DETAIL:  opt_param hint requires parameter name to be enclosed in single quotation marks.
LOG:  lt_hint_plan:
used hint:
not used hint:
duplication hint:
error hint:
opt_param(enable_hashjoin 'off')

                    QUERY PLAN                     
---------------------------------------------------
 Hash Join
   Hash Cond: (a.key1 = b.key1)
   ->  Seq Scan on test_opt_param1 a @"lt#0"
   ->  Hash
         ->  Seq Scan on test_opt_param2 b @"lt#0"
(5 rows)

EXPLAIN (COSTS false) select/*+opt_param('enable_hashjoin', off)*/ * from test_opt_param1 a join test_opt_param2 b on a.key1=b.key1;
INFO:  lt_hint_plan: hint syntax error at or near "opt_param('enable_hashjoin', off)"
DETAIL:  opt_param hint requires parameter values that are strings to be enclosed in single quotation mark.
LOG:  lt_hint_plan:
used hint:
not used hint:
duplication hint:
error hint:
opt_param('enable_hashjoin' off)

                    QUERY PLAN                     
---------------------------------------------------
 Hash Join
   Hash Cond: (a.key1 = b.key1)
   ->  Seq Scan on test_opt_param1 a @"lt#0"
   ->  Hash
         ->  Seq Scan on test_opt_param2 b @"lt#0"
(5 rows)