整型与浮点类型数据比较隐式转换规则

KingbaseES 数值类型有整型和浮点类型，如：integer , bigint 就是整型类型的，而numeric , real 则是浮点类型的数据。整型和浮点类型数据在进行比较时，需要有将其中一边数据类型进行转换，不恰当的类型转换会导致索引无法使用，影响SQL的执行性能。作为DBA，应当熟悉转换规则，主动采取转换，避免隐式转换。以下以例子的形式向大家展示KingbaseES 数据库 integer , numeric (decimal) , real (double) 三中类型数据相互比较时的类型转换规则。

一、创建测试数据

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create table test_integer(id integer,desc_info text); create table test_numeric(id numeric,desc_info text); create table test_real(id real,desc_info text);   insert into test_integer select generate_series(10001,20000),repeat('a',500); insert into test_numeric select generate_series(10001,20000),repeat('a',500); insert into test_real select generate_series(10001,20000),repeat('a',500);   create index ind_test_integer on test_integer(id); create index ind_test_numeric on test_numeric(id); create index ind_test_real on test_real(id);   analyze test_integer; analyze test_numeric; analyze test_real;

二、创建SQL游标

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prepare test_integer_bind_numeric(numeric) as select * from test_integer where id=$1; prepare test_integer_bind_real(real) as select * from test_integer where id=$1;   prepare test_numeric_bind_integer(integer) as select * from test_numeric where id=$1; prepare test_numeric_bind_real(real) as select * from test_numeric where id=$1;   prepare test_real_bind_numeric(numeric) as select * from test_real where id=$1; prepare test_real_bind_integer(integer) as select * from test_real where id=$1;

三、验证转换规则

1、integer 转成 numeric 

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testdb=# explain execute test_integer_bind_numeric(12345);                            QUERY PLAN                            -----------------------------------------------------------------  Seq Scan on test_integer  (cost=0.00..817.00 rows=50 width=508)    Filter: ((id)::numeric = '12345'::numeric) (2 rows)<br><br>testdb=# explain execute test_numeric_bind_integer(12345);                                       QUERY PLAN                                       ---------------------------------------------------------------------------------------  Index Scan using ind_test_numeric on test_numeric  (cost=0.29..8.30 rows=1 width=510)    Index Cond: (id = '12345'::numeric) (2 rows)

2、integer 转成 real

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testdb=# explain execute test_integer_bind_real(12345);                            QUERY PLAN                            -----------------------------------------------------------------  Seq Scan on test_integer  (cost=0.00..817.00 rows=50 width=508)    Filter: ((id)::double precision = '12345'::real) (2 rows)   testdb=# explain execute test_real_bind_integer(12345);                                    QUERY PLAN                                    ---------------------------------------------------------------------------------  Index Scan using ind_test_real on test_real  (cost=0.29..8.30 rows=1 width=508)    Index Cond: (id = '12345'::double precision) (2 rows)

3、numeric 转成 real

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testdb=# explain execute test_numeric_bind_real(12345);                            QUERY PLAN                            -----------------------------------------------------------------  Seq Scan on test_numeric  (cost=0.00..817.00 rows=50 width=510)    Filter: ((id)::double precision = '12345'::real) (2 rows)   testdb=# explain execute test_real_bind_numeric(12345);                                    QUERY PLAN                                    ---------------------------------------------------------------------------------  Index Scan using ind_test_real on test_real  (cost=0.29..8.30 rows=1 width=508)    Index Cond: (id = '12345'::double precision) (2 rows)

四、结论

integer , numeric , real 三种类型的数据在比较时，转换规则 integer -> numeric -> real 。