16402 | lib2 | 15936 | postgres | active | ::1 | select datid,datname,pid,usename,state,client_addr,query from pg_stat_activity; 16394 | lib1 | 2940 | postgres | idle | 127.0.0.1 | SELECT id,name,age,email FROM guser WHERE id=$1 16402 | lib2 | 18808 | postgres | idle | ...
In RDBMS, the NULL represents missing value or empty value. It is not the same as an empty string. Use the NOT NULL constraint on a column to restrict the NULL value in the column. The NOT NULL constraint can be declared with an individual column while creating a table or altering a ...
代价分析Bitmap Index Scan的代价估算就是Index Scan的访问索引数据的代价，即如下计算公式：SELECT round( current_setting('random_page_cost')::real * pages + current_setting('cpu_index_tuple_cost')::real * tuples + current_setting('cpu_operator_cost')::real * tuples) as bitmap_index_s...
select null = null;是的，但是我们没有得到任何结果。而不是=或!=，我们应该使用is或is not语句。s...
select null = null;是的，但是我们没有得到任何结果。而不是=或!=，我们应该使用is或is not语句。s...
select ename into v_arr(1) from emp where empno=&no; dbms_output.put_line('雇员名:'||v_arr(1)); end; 说明: table和varray的功能是相同的,都可以用来存储相同类型的多个数值; table和varray的使用方法是相同的,都需要先定义类型,再定义变量, ...
use the operatorIS NULLorIS NOT NULL.x is nullyields either true or false, it never yieldsnull...
Empty 当对应的缓冲池槽没有存储页面时(即refcount和usage_count为0),该描述符的状态为空。 Pinned 当相应的缓冲池槽存储一个页面并且任何 PostgreSQL 进程正在访问该页面(即 refcount 和 usage_count 大于或等于 1)时,该缓冲区描述符的状态被锁定。 Unpinned 当对应的缓冲池槽存储了一个页面但没有PostgreSQL...
Understanding NULL in PostgreSQL In PostgreSQL, NULL means no value. In other words, the NULL column does not have any value. It does not equal 0, empty string, or spaces. The NULL value cannot be tested using any equality operator like “=” “!=” etc. There are some special statemen...
接下来我们通过同一个SQL select * from t1 where a <= 10000; 的不同执行计划的代价,来理解扫描算子的过程。 顺序扫描 xiaoming=# explain select * from t1 where a <= 10000; QUERY PLAN --- Seq Scan on t1 (cost=0.00..35811.00 rows=20557 width=12) Filter: (a <= 10000)...