PostgreSQL源码分析——CREATE CAST

CREATE CAST源码分析

CREATE CAST用法

CREATE CAST —— 定义一个用户自定义的类型转换
用法如下：

CREATE CAST (source_type AS target_type)WITH FUNCTION function_name [ (argument_type [, ...]) ][ AS ASSIGNMENT | AS IMPLICIT ]CREATE CAST (source_type AS target_type)WITHOUT FUNCTION[ AS ASSIGNMENT | AS IMPLICIT ]CREATE CAST (source_type AS target_type)WITH INOUT[ AS ASSIGNMENT | AS IMPLICIT ]

如何使用以及用法请参考PostgreSQL文档中CREATE CAST一节。下面我们主要分析一下其源码，看一下是如何实现的。

源码分析

因为CREATE CAST属于Utility型语句，无需查询优化，其主流程如下：

exec_simple_query
--> pg_parse_query      // 语法解析
--> pg_analyze_and_rewrite   // 语义分析--> parse_analyze--> pg_rewrite_query
--> pg_plan_queries    // 生成执行计划
--> PortalStart
--> PortalRun          // 执行器执行--> PortalRunMulti--> PortalRunUtility--> ProcessUtility--> standard_ProcessUtility--> ProcessUtilitySlow--> CreateCast    // 进入CreateCast处理函数处理CREATE CAST语句
--> PortalDrop

主要的处理逻辑都在CreateCast函数中完成。后续会重点分析一下这个函数。

解析部分

我们首先分析一下其语法解析部分，这部分比较简单，核心是CreateCastStmt的定义，定义如下：

/* ----------------------*	CREATE CAST Statement* ----------------------*/
typedef struct CreateCastStmt
{NodeTag		type;TypeName   *sourcetype;TypeName   *targettype;ObjectWithArgs *func;CoercionContext context;bool		inout;
} CreateCastStmt;

CREATE CAST其在gram.y中定义的表示如下：

/***************************************************		CREATE CAST / DROP CAST**************************************************/
CreateCastStmt: CREATE CAST '(' Typename AS Typename ')'WITH FUNCTION function_with_argtypes cast_context{CreateCastStmt *n = makeNode(CreateCastStmt);n->sourcetype = $4;n->targettype = $6;n->func = $10;n->context = (CoercionContext) $11;n->inout = false;$$ = (Node *)n;}| CREATE CAST '(' Typename AS Typename ')'WITHOUT FUNCTION cast_context{CreateCastStmt *n = makeNode(CreateCastStmt);n->sourcetype = $4;n->targettype = $6;n->func = NULL;n->context = (CoercionContext) $10;n->inout = false;$$ = (Node *)n;}| CREATE CAST '(' Typename AS Typename ')'WITH INOUT cast_context{CreateCastStmt *n = makeNode(CreateCastStmt);n->sourcetype = $4;n->targettype = $6;n->func = NULL;n->context = (CoercionContext) $10;n->inout = true;$$ = (Node *)n;};cast_context:  AS IMPLICIT_P					{ $$ = COERCION_IMPLICIT; }| AS ASSIGNMENT							{ $$ = COERCION_ASSIGNMENT; }| /*EMPTY*/								{ $$ = COERCION_EXPLICIT; };DropCastStmt: DROP CAST opt_if_exists '(' Typename AS Typename ')' opt_drop_behavior{DropStmt *n = makeNode(DropStmt);n->removeType = OBJECT_CAST;n->objects = list_make1(list_make2($5, $7));n->behavior = $9;n->missing_ok = $3;n->concurrent = false;$$ = (Node *)n;};opt_if_exists: IF_P EXISTS						{ $$ = true; }| /*EMPTY*/								{ $$ = false; };

非常容易理解，下面我们重点分析一下CreateCast函数的实现。

执行部分

用户通过CREATE CAST语句自定义一个类型转换，数据库肯定有个地方将这个转换的信息存起来，这个地方就是pg_cast系统表。pg_cast系统表存在数据类型转换路径，包括内建和用户自定义的。

postgres@postgres=# \d pg_castTable "pg_catalog.pg_cast"Column    |  Type  | Collation | Nullable | Default 
-------------+--------+-----------+----------+---------oid         | oid    |           | not null | castsource  | oid    |           | not null |             -- 源数据类型的OIDcasttarget  | oid    |           | not null |             -- 目标数据类型的OIDcastfunc    | oid    |           | not null |             -- 执行该转换的函数的OID。如果该转换方法不需要一个函数则存储0。castcontext | "char" |           | not null |             -- 指示该转换能被调用的环境castmethod  | "char" |           | not null |             -- 指示转换如何被执行。
Indexes:"pg_cast_oid_index" UNIQUE, btree (oid)"pg_cast_source_target_index" UNIQUE, btree (castsource, casttarget)

而CreateCast函数的主要内容就是将用户自定义的类型转换信息插入到pg_cast系统表中。

CreateCast
--> LookupFuncWithArgs  // 查到pg_proc系统表，看是否已存在--> LookupFuncNameInternal--> FuncnameGetCandidates
--> IsBinaryCoercible(Oid srctype, Oid targettype) //Check if srctype is binary-coercible to targettype.
--> CastCreate  // 将类型转换信息插入pg_cast系统表中