now()函数是获取当前时间，当SQL语句中分区表时间传入now()时，由于SQL下推DN节点执行，优化器在CN节点生成执行计划时，无法进行分区剪枝，导致扫描了所有分区，导致SQL性能下降，SQL并行执行时会引起严重的分区表锁冲突。

针对此问题，建议传入具体值代替now()，或定义一个稳定函数代替now()。

以下是关于该场景的测试情况：

1. 准备测试数据：

   teledb=# create table t_time_range
   (f1 bigint, f2 timestamp ,f3 bigint) 
   partition by range (f2) begin (timestamp without time zone '2021-06-01 0:0:0') 
   step (interval '1 month') 
   partitions (12) distribute by shard(f1) 
   to group default_group;
   CREATE TABLE
   

2. 使用now()时SQL执行计划：

   可以看到，SQL被下推到DN节点执行，优化器没有剪枝，扫了所有分区。

   teledb=# explain select * from t_time_range where f2<now();
   QUERY PLAN 
   -------------------------------------------------------------------------------------------------------------------------------
   Remote Fast Query Execution (cost=0.00..0.00 rows=0 width=0)
   Node/s: dn001, dn002, dn003
   -> Append (cost=0.00..0.00 rows=0 width=0)
   -> Seq Scan on t_time_range (partition sequence: 0, name: t_time_range_part_0) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 1, name: t_time_range_part_1) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 2, name: t_time_range_part_2) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 3, name: t_time_range_part_3) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 4, name: t_time_range_part_4) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 5, name: t_time_range_part_5) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 6, name: t_time_range_part_6) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 7, name: t_time_range_part_7) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 8, name: t_time_range_part_8) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 9, name: t_time_range_part_9) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 10, name: t_time_range_part_10) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   -> Seq Scan on t_time_range (partition sequence: 11, name: t_time_range_part_11) (cost=0.00..2.17 rows=30 width=24)
   Filter: (f2 < now())
   (27 rows)
   

3. 自定义一个稳定函数

   teledb=# create or replace function get_current_timestamp() returns timestamp as
   $$
   begin
   return current_timestamp;
   end;
   $$
   language plpgsql IMMUTABLE;
   CREATE FUNCTION
   

4. 用自定义的稳定函数代替now()后的执行计划：

   可以看到，SQL下推DN节点执行后，优化器成功剪枝，只扫描了2个分区。

   teledb=# explain select * from t_time_range where f2<get_current_timestamp();
   QUERY PLAN 
   -------------------------------------------------------------------------------------------------------------------------------
   Remote Fast Query Execution (cost=0.00..0.00 rows=0 width=0)
   Node/s: dn001, dn002, dn003
   -> Append (cost=0.00..0.00 rows=0 width=0)
   -> Seq Scan on t_time_range (partition sequence: 0, name: t_time_range_part_0) (cost=0.00..11.69 rows=178 width=24)
   Filter: (f2 < '2021-07-12 11:31:33.639119'::timestamp without time zone)
   -> Seq Scan on t_time_range (partition sequence: 1, name: t_time_range_part_1) (cost=0.00..11.69 rows=178 width=24)
   Filter: (f2 < '2021-07-12 11:31:33.639119'::timestamp without time zone)
   (7 rows)