[Basic introduction to postgresql] Introduction to data types, integers, strings, floating point numbers, date and time type characteristics, precision and representation range, choose the appropriate type to improve performance

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Overview

As an open source database widely used in various fields, the postgresql database has rich data types. Like other programming languages, when you start using a programming language, you must first understand its data types and be able to match them one by one with data expressed in natural language. Correspond.

This article will share with you the existing data types of postgresql and provide a detailed introduction to some commonly used data types.

Type overview

There are about 43 types listed in the postgresql manual, the list is as follows:

Type name	Alias	describe
bigint	you8	Signed integer, occupies 8 bytes
bigserial	serial8	Auto-incrementing 8-byte integer
bit [ (n) ]		Fixed length binary bit string
bit varying [ (n) ]	wander [ (n) ]	Variable-length binary bit string
boolean	bool	Boolean type (true/false)
box		rectangular box on a plane
bytea		Binary data, similar to an array of binary bits
character [ (n) ]	char [ (n) ]	Fixed length string type
character varying [ (n) ]	varchar [ (n) ]	Variable length string type
cidr		IPv4 or IPv6 network address
circle		circle on a plane
date		calendar date (year, month, day)
double precision	float8	Double precision floating point number, 8 bytes
inet		IPv4 or IPv6 host address
integer	int, int4	Signed 4-byte integer
interval [ fields ] [ § ]		time span
json		textual JSON data
jsonb		binary JSON data, decomposed
line		infinite line on a plane
lsg		line segment on a plane
macadr		MAC (Media Access Control) address
Macaddr8		MAC (Media Access Control) address (EUI-64 format)
money		currency amount
numeric [ (p, s) ]	decimal [ (p, s) ]	Type of precision that can be specified
path		geometric path on a plane
pg_lsn		PostgreSQL Log Sequence Number
pg_snapshot		user-level transaction ID snapshot
point		geometric point on a plane
polygon		closed geometric path on a plane
real	float4	Single precision floating point number occupies 4 bytes
smallint	int2	Signed two-byte integer
smallserial	serial2	self-increasing two-byte integer type
serial	serial4	self-increasing 4-byte integer
text		variable length string type
time [ § ] [ without time zone ]		time of day (no time zone)
time [ § ] with time zone	timetz	time of day, including time zone
timestamp [ § ] [ without time zone ]		date and time (no time zone)
timestamp [ § ] with time zone	timestamptz	date and time, including time zone
tsquery		text search query
tsvector		text search document
txid_snapshot		user-level transaction ID snapshot (deprecated; see pg_snapshot)
uuid		Globally unique identifier
xml		XML data type

There are some commonly used ones, such as int, varchar, etc., as well as special types such as pg_lsn that are not commonly used. The following is a detailed description of several commonly used types.

integer type

The integer type is used to store an integer, which is divided into the following types according to the bytes stored:

smallint, occupies 2 bytes, is a signed type, the minimum value is -2 ¹⁵ , the maximum value is 2 ¹⁵ -1
integer, 4 bytes, signed type, minimum value -2 ³¹ , maximum value 2^31-1
bigint, 8 bytes, signed type, minimum value -2 ⁶³ , maximum value 2 ⁶³ -1

There is no unsigned type provided here. When the stored data exceeds the type range, an error will be reported.
When selecting a type, you should choose an appropriate type based on the range of your data. If you choose a type with too large a range, it will not only take up too much space, but also affect the performance of data reading and writing.

floating point type

There are three main storage types of floating point values in postgresql:

real, corresponding to the single-precision type, occupies 4 bytes, and can be recorded in the range of 1E-37 to 1E+37. The maximum precision is 6 digits after the decimal point, and the 7th digit is an approximate value;
Double precision, for the double precision type, occupies 8 bytes, and can be recorded in the range of 1E-307 to 1E+308. The maximum precision is 15 digits after the decimal point, and the same 16 digits are approximate values;
numeric(precision,scale), arbitrary precision type, precision specifies the total number of significant digits, and scale specifies the number of decimal places;

In postgresql, which is compatible with the SQL standard, there is also a float type, written as float(n), n can specify the number of binary digits of precision. When written as float, the default corresponds to the double precision type; float(1) to float(24) correspond to the single-precision real type, and float(25) to float(53) correspond to the double precision type; in fact, according to the above three basic type processing.

Floating point type values, in addition to normal numbers, also define three special values:

Infinity，正的无穷大；无穷大这个数字是数学中的一个概念，符合推理：无穷 + 无穷=无穷；无穷-无穷=NaN；
-Infinity，负的无穷大；
NaN，not a number,不是一个数字；在IEEE 754标准中，NaN不等与其它数比较，包括自己；但是在postgresql中为了索引处理方便，两个NaN是相等的，同时NaN大于任何非NaN值。

下面通过实践来看一下以上用法：

先来创建一张表，含有三种基本类型：

postgres=# create table floating(a real,b double precision, c numeric(10,6));
CREATE TABLE

插入数据，并且含有特殊的值：

postgres=# insert into floating values('Infinity','NaN',10.2023333);
INSERT 0 1
postgres=# insert into floating values(2.8,1.0,1555555.2023333);
ERROR:  numeric field overflow
DETAIL:  A field with precision 10, scale 6 must round to an absolute value less than 10^4.
postgres=# insert into floating values(2.8,1.0,155.2023333);
INSERT 0 1
postgres=# select * from floating ;
    a     |  b  |     c
----------+-----+------------
 Infinity | NaN |  10.202333
      2.8 |   1 | 155.202333
(2 rows)

在插入数据时，有一个报错，超过了numeric(10,6)的范围，可以看到它不会自动截短处理；
查询可以看到特殊值也可以显示出来。

下面我们进行几个条件判断的处理：

postgres=# select * from floating where b > 10;
    a     |  b  |     c
----------+-----+-----------
 Infinity | NaN | 10.202333
(1 row)

postgres=# select * from floating where a < 'NaN';
    a     |  b  |     c
----------+-----+------------
 Infinity | NaN |  10.202333
      2.8 |   1 | 155.202333
(2 rows)

可以看到第一条查询，NaN是大于任何值的，从第二条查询可以看出NaN居然是大于无穷大的一个特殊值，所以在使用时要特别注意。

字符类型

字符类型几乎无处不在，在postgresql提供了以下几种类型：

character(n),也可以写作char(n)，它是定长的类型，也就是占用的存储空间是固定大小，大小由n来指定，它是正整数；当不指定n时，占一个字符，等同与char(1);
character varying(n),也可以写作varchar(n)，它是变长的类型，也就是占用的存储空间是实际字符串的长度，而n用来限制最大长度；
text，变长类型，不限制字符串的长度；

这里需要特别说明几点：

n 的取值最大值为 10485760，也就是十六进制的0xA00000；
varchar 不指定n时，与text相同，而指定n时，只是会增加长度的检查；
在postgresql 中这三种字符类型的性能相同，而后两者更省空间；
对于超出n限制的字符串，当在显示类型转换到字符串时，会自动截断；而当字符串中超出部分都为空格时，也会自动将尾部连续空格截断；

下面举例来说明。

超过最大值

n的最大值限制为0xA00000，如果超过就会失败。

postgres=# create table ch(sname char(1070596096));
ERROR:  length for type char cannot exceed 10485760
LINE 1: create table ch(sname char(1070596096));

三种类型比较

首先创建含有三种类型的表str，其中char实际存储一个字节；

postgres=# create table str(a char,b varchar(5),c text);
CREATE TABLE

插入符合长度的字符串，可以看到text没有限制。

postgres=# insert into str values('1','12345','123456789');
INSERT 0 1

插入尾部带有空格的字符串，非空格字符长度不超过限制，可以看到空格会被截断，也会插入成功；

postgres=# insert into str values('2   ','12345   ','1234    56789');
INSERT 0 1
postgres=# select * from str;
 a |   b   |       c
---+-------+---------------
 1 | 12345 | 123456789
 2 | 12345 | 1234    56789
(2 rows)

postgres=# insert into str values('3   ',' 32345   ','1234    56789');
ERROR:  value too long for type character varying(5)

而对于字符中间的空格，也会被算为有效字符串中，所以再次在字符头部加入空格，就会超过varchar(5)的长度限制。

布尔类型

布尔类型也是我们常用的类型，经常表示两种状态，它实际存储时会有三种值的情况：

true，为真时的情况；
false，为假时的情况；
NULL, 没有赋值时的情况；

在给布尔类型赋值时，也有多种写法：

true，也可以用’1’，’true’,‘t’,‘yes’,‘y’ 一共6种写法；
false, 也可以用’0’,‘false’,‘f’,‘no’,‘n’ 对应的也有6种写法；

当然在实际应用中，整体采用一种对应的写法规则，避免开发的混乱；
下面我们来举一个例子；

postgres=# create table switchState(sid int primary key, sstate boolean);
CREATE TABLE
postgres=# insert into switchstate values(1,true),(2,'true'),(3,'t'),(4,'no'),(5,'n'),(6,'0');
INSERT 0 6
postgres=# select * from switchstate ;
 sid | sstate
-----+--------
   1 | t
   2 | t
   3 | t
   4 | f
   5 | f
   6 | f
(6 rows)

可以看到，不管我们用什么写法，数据库中只显示t和f。

为了避免有NULL的情况出现时，可以设置boolean类型的默认值，没有赋值时就会采用默认值。

postgres=# drop table switchstate ;
DROP TABLE
postgres=# create table switchState(sid int primary key, sstate boolean default false);
CREATE TABLE
postgres=# insert into switchstate values(1);
INSERT 0 1
postgres=# select * from switchstate ;
 sid | sstate
-----+--------
   1 | f
(1 row)

日期时间类型

日期时间是经常会用到的，因为不同历法，时间计法不同，以及显示方式的不同，变化非常多样，本文作为基础入门，只列出几种常用的用法，不再深入探讨。

与日期时间相关的类型有：

timestamp with time zone，含有日期和时间，占8个字节，带有时区；最小精度到微秒，可以表示的日期范围为公元前4713年到公元294276年，非常遥远；
timestamp [without time zone]，与上一类型一样，只是不带时区；
date, 日期类型，表示年月日，占4个字节；默认格式为yyyy-mm-dd, 最小单位为天，可以表示的日期范围为公元前4713年到公元294276年；
time with time zone ，表示一天中的时间，不带日期；占12个字节，默认格式为HH:MI:SS，表示范围为00:00:00到24:00:00，最小精度到微秒，同样带有时区；
time [without time zone]，与上一类型一样，只是不带时区，所以占8字节；
interval，时间间隔类型，占16字节，间隔时间范围可以为-178000000年到178000000年，最小精度为微秒；

下面我们来使用一下日期和时间的类型，首先创建一张表，带有日期，日期时间，时间类型。

postgres=# create table tbl_datetime(day date, year timestamp with time zone, hour time);
CREATE TABLE
postgres=# insert into tbl_datetime values(now(), now(), CURRENT_TIME);
INSERT 0 1
postgres=# select * from tbl_datetime ;
    day     |             year              |      hour
------------+-------------------------------+-----------------
 2024-03-13 | 2024-03-13 22:53:13.555858+08 | 22:53:13.555858
(1 row)

然后插入数据，这里使用了now()来获取当前的日期和时间，用CURRENT_TIME获取当前的时间；

查询可以看到默认格式的显示，当前时区为东8区。

总结

在本章节中，介绍了postgresql数据库中的数据类型，同时详细对常用的整型，浮点，字符串，日期时间类型进行了介绍，占用的存储空间，表示的范围，以及它们的精度，尤其对于字符类型，最好采用变长的类型来减少存储空间，从而提升查询效率。

六、结尾

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