Postgresql與MySQL的區別

在學習具體的Postgresql用法以前，咱們來思考一下Postgresql與目前最經常使用的關係型數據庫MySQL的區別在哪裏，主要集中在二者的底層結構、使用場景和特別之處。

1.前言

當咱們去選擇項目使用的數據庫的時候，應該要考慮該項目的應用場景、數據規模等因素。每一種數據庫都有它特定的應用場景，好比咱們如今要討論的Postgresql和MySQL數據庫，這兩種的數據庫的應用場景就有所不一樣，下面咱們就它們的相同點和不一樣點進行討論。

2.Postgresql vs MySQL

此表來源於postgresqltutorial，詳細區別了二者的不一樣點。

特性	Postgresql	MySQL
描述	The world’s most advanced open source database	The world’s most popular open source database
發展	PostgreSQL is an open source project	MySQL is an open-source product
實現語言	C	C、C++
圖形化工具	PgAdmin	MySQL Workbench
ACID	Yes	Yes
存儲引擎	Single storage engine	Multiple storage engines e.g., InnoDB and MyISAM
全文檢索	Yes	Yes
Drop a temporary table（刪除一個臨時表）	No TEMP or TEMPORARY keyword in DROP TABLE statement（隨着數據庫的鏈接的斷開而被刪除）	MySQL supports the TEMP or TEMPORARYkeyword in the DROP TABLE statement that allows you to remove the temporary table only.（須要手動刪除）
DROP TABLE（刪除表）	Support CASCADE option to drop table’s dependent objects e.g., tables, views, etc.,（級聯操做：也就是更新、刪除父表，將會同步更新、刪除子表；而反過來則不變）	Does not support CASCADE option
TRUNCATE TABLE（刪除表）	PostgreSQL TRUNCATE TABLEsupports more features like CASCADE, RESTART IDENTITY, CONTINUE IDENTITY, transaction-safe, etc.（對於移除表中的數據，delete是能夠的，可是對於一個大表，truncate是更加有效的方式，由於truncate刪除表中全部行的時候不須要掃表整個表）	MySQL TRUNCATE TABLE does not support CASCADE and transaction safe i.e,. once data is deleted, it cannot be rolled back.（永久性刪除，不能夠撤銷）
自動增長列	SERIAL	AUTO_INCREMENT
解析功能	Yes	No
Data types	Support many advanced types such as array, hstore, and user-defined type.	SQL-standard types
Unsigned integer	No	Yes
Boolean type	Yes	Use TINYINT(1) internally for Boolean
IP address data type	Yes	No
設置列默認值	Support both constant and function call	Must be a constant or CURRENT_TIMESTAMP for TIMESTAMP or DATETIME columns
CTE（Common Table Expressions）	Yes	No
EXPLAINoutput	More detailed	Less detailed
Materialized views（物化視圖）	Yes（Postgresql將視圖概念擴展到下一個級別，容許視圖在物理上存儲數據，咱們將這些視圖稱爲物化視圖，物化視圖會緩存複雜的查詢結果，而後容許按期刷新此結果）	No
CHECK constraint（檢查約束）	Yes	No (MySQL ignores the CHECK constraint)
Table inheritance（表繼承）	Yes	No
Programming languages for stored procedures	Ruby, Perl, Python, TCL, PL/pgSQL, SQL, JavaScript, etc.	SQL:2003 syntax for stored procedures
FULL OUTER JOIN（全外鏈接）	Yes	No
INTERSECT	Yes（Postgresql的INTERSECT運算符將兩個或多個SELECT語句的結果集合併到一個結果集中）	No
EXCEPT	Yes（Except運算符經過比較兩個或多個quires的結果集來返回行，此返回行存在於第一查詢子句而不存在第二查詢子句中）	No
Partial indexes（部分索引）	Yes	No
Bitmap indexes（位圖索引）	Yes	No
Expression indexes（表達式索引）	Yes	NO
Covering indexes（覆蓋索引）	Yes (since version 9.2)例子1、例子2	Yes. MySQL supports covering indexes that allow data to be retrieved by scanning the index alone without touching the table data. This is advantageous in case of large tables with millions of rows.
Common table expression (CTE)	Yes	Yes. (since version 8.0, MySQL has supported CTE)
Triggers（觸發器）	Support triggers that can fire on most types of command, except for ones affecting the database globally e.g., roles and tablespaces.	Limited to some commands
Partitioning（分區）	RANGE, LIST	RANGE, LIST, HASH, KEY, and composite partitioning using a combination of RANGE or LIST with HASH or KEY subpartitions
Task Schedule（任務定時）	pgAgent	Scheduled event
Connection Scalability（鏈接規模）	Each new connection is an OS process（進程）	Each new connection is an OS thread（線程）
SQL compliant（SQL兼容性）	PostgreSQL is largely SQL compliant.	MySQL is partially SQL compliant. For example, it does not support check constraint.
Best suited	PostgreSQL performance is utilized when executing complex queries.	MySQL performs well in OLAP& OLTP systems when only read speeds are needed.
Support for JSON	Support JSON and other NoSQL features like native XML support. It also allows indexing JSON data for faster access.	MySQL has a JSON data type support but does not support any other NoSQL feature.
Default values	The default values can be changed at the system level only	The default values can be overwritten at the session level and the statement level
B-tree Indexes	B-tree indexes merged at runtime to evaluate are dynamically converted predicates.	Two or more B-tree indexes can be used when it is appropriate.
Object statistics	Very good object statistics	Fairly good object statistics

對於上面有部分疑惑的點進行詳細分析：

一、Full outer join全外鏈接

做用：Postgresql full outer join返回來自兩個參與表的全部行，若是他們在相對的表上沒有匹配，則使用null填充。full outer join組合了左外連接和右外鏈接的結果，並返回鏈接子句兩側表中的全部行（匹配或者不匹配）行。

3.Disadvantages of using MySQL（MySQL的缺點）

• Transactions related to system catalog are not ACID compliant（與系統目錄相關的事務不符合ACID）
• Some time A server crash can corrupt the system catalog（有時服務器崩潰可能會破壞系統目錄）
• No pluggable authentication module preventing centrally managed account（沒有可插入的身份驗證模塊阻止集中管理帳戶）
• No support for roles so it is difficult in maintaining privileges for many users（不支持角色，所以很難爲許多用戶維護權限）
• Stored procedures are not cacheable（存儲過程不可緩存）
• Tables used for the procedure or trigger are always pre-locked（用於程序或觸發器的表始終是預先鎖定的）

4.Disadvantages of using PostgreSQL（Postgresql的缺點）

• The current external solutions require a high learning curve（當前的外部解決方案須要很高的學習曲線）
• No upgrade facility for major releases（沒有主要版本的升級工具）
• The data need to be exported or replicated to the new version（須要將數據導出或複製到新版本）
• Double storage is needed during the upgrade process（升級過程當中須要雙重存儲）
• indexes cannot be used to directly return the results of a query（索引不能用於直接返回查詢結果）
• Query execution plans are not cached（不緩存查詢執行計劃）
• Bulk loading operations may become CPU bound（批量加載操做可能會受CPU限制）
• Sparse Independent Software Vendor support（稀疏獨立軟件供應商支持）

5.What is Better?

After comparing both we can say that MySQL has done a great job of improving itself to keep relevant, but on the other side for PostgreSQL, you don't need any licensing. It also offers table inheritance, rules systems, custom data types, and database events. So, it certainly edges above MySQL.

6.參考資料

hackr.io/blog/postgr…

www.guru99.com/postgresql-…