PostgreSQL源码分析(2)– 常用数据类型/SQL语句的解释和执行

PostgreSQL源码分析(2)– 常用数据类型/SQL语句的解释和执行
出处：http://www.loveopensource.com/?p=27

主要分析文件：
// basic nodes definition
src/include/nodes/nodes.h
// SQL parsed struct
src/include/nodes/parsenodes.h
// List定义
src/include/nodes/pg_list.h
// List实现
src/backend/nodes/list.c
// postgres运行入口文件
src/backend/tcop/postgres.c
// utility Stmt运行文件
src/backend/tcop/utility.c
// SQL analyze and rewrite
src/backend/parser/analyze.c

PostgreSQL用一种非常简单的形式实现了类似C++的继承,请看nodes.h ：
typedef struct Node
{
NodeTag type;
} Node;
然后请看:parsenodes.h(SQL语句经过parser解析后都先对应该该文件的一个struct中)
假设有一个create table的sql:
create table test (name varchar(100, pass varchar(100));

将会被解析到如下structure:
typedef struct CreateStmt
{
NodeTag type;
RangeVar *relation; /* relation to create */
List *tableElts; /* column definitions (list of ColumnDef) */
List *inhRelations; /* relations to inherit from (list of
* inhRelation) */
List *constraints; /* constraints (list of Constraint nodes) */
List *options; /* options from WITH clause */
OnCommitAction oncommit; /* what do we do at COMMIT? */
char *tablespacename; /* table space to use, or NULL */
} CreateStmt;

首先，看看该struct的第一个元素：type是一个NodeTag类型。
PG的很多struct的第一个元素都是NodeTag，这样在函数中传递指针变量时，他可以很
简单的把参数设置成：Node*
说简单点，其实有点像是所有的struct都继承了Node这个struct.
就是因为这个原因，看PG的代码很累，很多函数的参数和返回值都是一个简单的Node.
在nodes.h中有每个Node的值的定义,比如：上面说的CreateStmt的type的值就是：T_CreateStmt

然后，PG中大量的使用了链表类型:List
在pg_list.h中有定义：
typedef struct List
{
NodeTag type; /* T_List, T_IntList, or T_OidList */
int length;
ListCell *head;
ListCell *tail;
} List;

可以看到，List的定义是基于基类Node来进行的。
常用的List操作函数有：
//取List第一个元素
ListCell *y = list_head(List *l);
//得到List的元素个数
list_length(List *l);
// 遍历List
foreach(cell, l)
{
…
}
其他的很多函数具体可以参考pg_list.h和list.c
下面接着说SQL的解释和执行。
所有的SQL都会先解析成一个与之相对应的struct.
Select会解析到:
typedef struct SelectStmt
{
NodeTag type;

/*
* These fields are used only in “leaf” SelectStmts.
*
* into, intoColNames, intoOptions, intoOnCommit, and intoTableSpaceName
* are a kluge; they belong somewhere else…
*/
List *distinctClause; /* NULL, list of DISTINCT ON exprs, or
* lcons(NIL,NIL) for all (SELECT DISTINCT) */
RangeVar *into; /* target table (for select into table) */
List *intoColNames; /* column names for into table */
List *intoOptions; /* options from WITH clause */
OnCommitAction intoOnCommit; /* what do we do at COMMIT? */
char *intoTableSpaceName; /* table space to use, or NULL */
List *targetList; /* the target list (of ResTarget) */
List *fromClause; /* the FROM clause */
Node *whereClause; /* WHERE qualification */
List *groupClause; /* GROUP BY clauses */
Node *havingClause; /* H***ING conditional-expression */

/*
* In a “leaf” node representing a VALUES list, the above fields are all
* null, and instead this field is set. Note that the elements of the
* sublists are just expressions, without ResTarget decoration. Also note
* that a list element can be DEFAULT (represented as a SetToDefault
* node), regardless of the context of the VALUES list. It’s up to parse
* analysis to reject that where not valid.
*/
List *valuesLists; /* untransformed list of expression lists */

/*
* These fields are used in both “leaf” SelectStmts and upper-level
* SelectStmts.
*/
List *sortClause; /* sort clause (a list of SortBy’s) */
Node *limitOffset; /* # of result tuples to skip */
Node *limitCount; /* # of result tuples to return */
List *lockingClause; /* FOR UPDATE (list of LockingClause’s) */

/*
* These fields are used only in upper-level SelectStmts.
*/
SetOperation op; /* type of set op */
bool all; /* ALL specified? */
struct SelectStmt *larg; /* left child */
struct SelectStmt *rarg; /* right child */
/* Eventually add fields for CORRESPONDING spec here */
} SelectStmt;

Delete会解析到：
typedef struct DeleteStmt
{
NodeTag type;
RangeVar *relation; /* relation to delete from */
List *usingClause; /* optional using clause for more tables */
Node *whereClause; /* qualifications */
List *returningList; /* list of expressions to return */
} DeleteStmt;

Update会解析到:
typedef struct UpdateStmt
{
NodeTag type;
RangeVar *relation; /* relation to update */
List *targetList; /* the target list (of ResTarget) */
Node *whereClause; /* qualifications */
List *fromClause; /* optional from clause for more tables */
List *returningList; /* list of expressions to return */
} UpdateStmt;

从定义上看，Select比较复杂。其实在PG内部，把Select/Delete/Update当成一样处理，只是最后
找到相应的结果集时采取不同的操作。
postgres.c的804行可以看到这一步操作：
parsetree_list = pg_parse_query(query_string);
第一步完成后，只是做了很简单、很粗糙的事情，然后，要进一步进行rewrite, 在交给优化器进行优化和
路径选择之前，所有的执行语句都要转换成struct Query:
typedef struct Query
{
NodeTag type;

CmdType commandType; /* select|insert|update|delete|utility */
/*
注意：
如果commandType为: utility，优化器不会对该SQL进行进一步优化，因为这个SQL
就是一些建表或者其他命令操作，无法进行路径选择和优化，这时候，executor直接
执行utilityStmt这个Node对应的Struct.

对于select|insert|update|delete这些SQL，优化器都需要进行评估和优化。

*/
QuerySource querySource; /* where did I come from? */

bool canSetTag; /* do I set the command result tag? */

Node *utilityStmt; /* non-null if this is a non-optimizable
* statement */

int resultRelation; /* rtable index of target relation for
* INSERT/UPDATE/DELETE; 0 for SELECT */

RangeVar *into; /* target relation for SELECT INTO */
List *intoOptions; /* options from WITH clause */
OnCommitAction intoOnCommit; /* what do we do at COMMIT? */
char *intoTableSpaceName; /* table space to use, or NULL */

bool hasAggs; /* has aggregates in tlist or havingQual */
bool hasSubLinks; /* has subquery SubLink */

List *rtable; /* list of range table entries */
FromExpr *jointree; /* table join tree (FROM and WHERE clauses) */

List *targetList; /* target list (of TargetEntry) */

List *returningList; /* return-values list (of TargetEntry) */

List *groupClause; /* a list of GroupClause’s */

Node *havingQual; /* qualifications applied to groups */

List *distinctClause; /* a list of SortClause’s */

List *sortClause; /* a list of SortClause’s */

Node *limitOffset; /* # of result tuples to skip (int8 expr) */
Node *limitCount; /* # of result tuples to return (int8 expr) */

List *rowMarks; /* a list of RowMarkClause’s */

Node *setOperations; /* set-operation tree if this is top level of
* a UNION/INTERSECT/EXCEPT query */

/*
* If the resultRelation turns out to be the parent of an inheritance
* tree, the planner will add all the child tables to the rtable and store
* a list of the rtindexes of all the result relations here. This is done
* at plan time, not parse time, since we don’t want to commit to the
* exact set of child tables at parse time. XXX This field ought to go in
* some sort of TopPlan plan node, not in the Query.
*/
List *resultRelations; /* integer list of RT indexes, or NIL */

/*
* If the query has a returningList then the planner will store a list of
* processed targetlists (one per result relation) here. We must have a
* separate RETURNING targetlist for each result rel because column
* numbers may vary within an inheritance tree. In the targetlists, Vars
* referencing the result relation will have their original varno and
* varattno, while Vars referencing other rels will be converted to have
* varno OUTER and varattno referencing a resjunk entry in the top plan
* node’s targetlist. XXX This field ought to go in some sort of TopPlan
* plan node, not in the Query.
*/
List *returningLists; /* list of lists of TargetEntry, or NIL */
} Query;

这些rewrite比较复杂，是由一系列的transform函数完成的，具体可以查看analyze.c.

下面把一些Sql命令转化后的struct做一个简单的分析。
首先分析utilityStmt, 这些命令都比较简单。
(1) Create Table

/* ———————-
* Create Table Statement
*
* NOTE: in the raw gram.y output, ColumnDef, Constraint, and FkConstraint
* nodes are intermixed in tableElts, and constraints is NIL. After parse
* analysis, tableElts contains just ColumnDefs, and constraints contains
* just Constraint nodes (in fact, only CONSTR_CHECK nodes, in the present
* implementation).
* ———————-
*/

typedef struct CreateStmt
{
// type应该为 T_CreateStmt
NodeTag type;
// relation->relname 就是要创建的table名字
RangeVar *relation; /* relation to create */
List *tableElts; /* column definitions (list of ColumnDef) */
// 由于PG的表是可以继承的，这部分先不分析
List *inhRelations; /* relations to inherit from (list of
* inhRelation) */
// 这些都是constraints的定义
List *constraints; /* constraints (list of Constraint nodes) */
List *options; /* options from WITH clause */
OnCommitAction oncommit; /* what do we do at COMMIT? */
char *tablespacename; /* table space to use, or NULL */
} CreateStmt;

其中最重要的就是：tableElts这个字段，里面包含了要创建table的所有columns.
我写了几行代码，可以遍历这一List.

List *schema;
ListCell *col;

elog(LOG, “begin create:%s”, ((CreateStmt *)parsetree)->relation->relname);
schema = ((CreateStmt *)parsetree)->tableElts;
// 遍历
foreach(col, schema)
{
ColumnDef *entry = lfirst(col);
elog(LOG, “column name:%s column type:%s”,
entry->colname, TypeNameToString(entry->typename));
}

建表语句：
pgsql=# create table lijianghua ( name varchar(100), pass varchar(100));
CREATE TABLE

log输出：
LOG: begin create:lijianghua
LOG: column name:name column type:pg_catalog.varchar
LOG: column name:pass column type:pg_catalog.varchar
(2) drop object
删除一个对象时，比如：drop table/drop view……, 命令会被解析到DropStmt struct:
/* ———————-
* Drop Table|Sequence|View|Index|Type|Domain|Conversion|Schema Statement
* ———————-
*/
typedef struct DropStmt
{
NodeTag type;
// 需要删除的对象列表
List *objects; /* list of sublists of names (as Values) */
// 对象类型
ObjectType removeType; /* object type */
DropBehavior behavior; /* RESTRICT or CASCADE behavior */
bool missing_ok; /* skip error if object is missing? */
} DropStmt;

ObjectType的定义也非常简单：
typedef enum ObjectType
{
OBJECT_AGGREGATE,
OBJECT_CAST,
OBJECT_COLUMN,
OBJECT_CONSTRAINT,
OBJECT_CONVERSION,
OBJECT_DATABASE,
OBJECT_DOMAIN,
OBJECT_FUNCTION,
OBJECT_INDEX,
OBJECT_LANGUAGE,
OBJECT_LARGEOBJECT,
OBJECT_OPCLASS,
OBJECT_OPERATOR,
OBJECT_ROLE,
OBJECT_RULE,
OBJECT_SCHEMA,
OBJECT_SEQUENCE,
OBJECT_TABLE,
OBJECT_TABLESPACE,
OBJECT_TRIGGER,
OBJECT_TYPE,
OBJECT_VIEW
} ObjectType;
同样，我也写了几行代码，进行了相关验证：
if(parsetree->type == T_DropStmt)
{
List *tables;
ListCell *table;
tables = ((DropStmt *)parsetree)->objects;
foreach(table, tables)
{
List *names = (List *) lfirst(table);
RangeVar *rel;
if(((DropStmt *)parsetree)->removeType == OBJECT_TABLE)
{
rel = makeRangeVarFromNameList(names);
elog(LOG, “want to delete table:%s”, rel->relname);
}
}
}

SQL语句：
pgsql=# drop table test,test21;
DROP TABLE

log输出：
LOG: want to delete table:test
LOG: want to delete table:test21

(3) 把比较复杂的Insert/Update/Delete/Select做一些详细的分析