PostgreSQL服务过程中的那些事二：Pg服务进程处理简单查询六：执行器执行
话说查询“select cname, comp from test1, test2 wheretest1.id=test2.id;” 发送到服务器端，走查询分支exec_simple_query，先调用start_xact_command初始化了事务管理相关对象和资源，接着调用pg_parse_query，通过Lex和Yacc对传入SQL语句进行词法语法解析，生成解析树。下来调用GetTransactionSnapshot方法做内存快照，然后调用pg_analyze_and_rewrite方法，进行语义分析把parsetree转换成querytree，然后对该querytree进行重写。接着调用pg_plan_queries方法，根据querytree做查询规划，生成查询计划树plantree。然后调用了一系列方法CreatePortal、PortalStart、PortalSetResultFormat、CreateDestReceiver、PortalRun、PortalDrop，创建portal、初始化portal、设置结果列格式、创建目的地接收者、运行portal执行查询并返回结果，然后释放portal相关资源，再调用finish_xact_command释放事务相关资源。这个例子终于执行完了。1
下面是执行plantree的调用序列图。
执行plantree的调用序列图
上图红色方框中显示了执行plantree的过程，主要分为portal创建和初始化（CreatePortal、PortalStart）、执行（PortalRun）、释放资源（PortalDrop）三个部分。portal是个抽象概念，它表示一个正在运行或可运行query的执行状态。Portal支持SQL层的CURSOR和协议层的portal。从上图中可以看出，在各个部分，portal调用了执行器executor的对应方法ExecutorStart、ExecutorRun、ExecutorDrop完成了相关操作。
简单描述一下执行过程，方法CreatePortal在内存上下文PortalMemory里创建portal，并在portal管理器"Portal hash"中注册（参见《PostgreSQL服务过程中的那些事一：启动postgres服务进程一.七：初始化portal管理环境》）；调用PortalDefineQuery为新建的portal设置sourceText（就是例子里的源SQL语句）、stmts（就是上一节讨论的plantree列表）等字段，将portal状态设置为PORTAL_DEFINED；再调用PortalStart初始化portal，在该例子里选择PORTAL_ONE_SELECT策略，调用CreateQueryDesc方法，为portal创建查询描述符，将portal状态设置为PORTAL_READY；调用方法PortalRun，根据选择策略调用相应方法，根据portal相关由plantree转化来的各节点结构由下至上逐层进行处理，涉及扫描数据、进行投射、结果缓存等；最后调用PortalDrop释放portal相关资源。
portal有多个执行策略，依赖于执行什么样的query（其实就是根据不同的SQL语句调用不同的方法以完成相应操作）。（注意：在所有情况下，一个portal仅执行一个源SQL query，并且从用户的视点，仅产生一个结果。但是，规则重写器可以扩展一个源query成0或多个实际的query。）下面是portal的四种策略
PORTAL_ONE_SELECT：这个portal包含一个SELECT查询。我们增加运行一个执行器作为被要求的结果。这个策略还支持可持有的cursor（为了事务结束后访问结果，执行器结果可以被转储到一个tuplestore里）。
PORTAL_ONE_RETURNING：这个portal包含一个伴有RETURNING子句的INSERT/UPDATE/DELETE查询（由规则重写器重写增加的可能的辅助查询/query）。在第一次执行时，我们运行portal以完成和转储主query的结果到portal的tuplestore里；然后这个结果按要求返回给用户。（我们不支持query的部分遍历悬挂，因为AFTER触发器代码不能处理，并且还因为我们不想在执行所有辅助query时冒失败风险。）
PORTAL_ONE_MOD_WITH： portal包含一个SELECT qiery，但它包含数据修改CTE。这个目前和处理PORTAL_ONE_RETURNING的情况一样，因为需要触发触发器的可能性。将来处理这种情况的行为可以更像PORTAL_ONE_SELECT。
PORTAL_UTIL_SELECT：portal包含一个utility语句，其返回一个象SELECT那样的结果（例如，WXPLAIN或者SHOW）。在第一次执行时，我们运行这个语句任何转储其结果到portal的tuplestore里；然后这个结果按要求的返回给客户端。
PORTAL_MULTI_QUERY：所有其它情况。这儿，我们不支持portal执行：portal的query会被运行以完成第一次调用。
下面是portal和其执行状态、策略的类型定义及执行器状态结构定义，其他涉及到节点结构定义略去。
typedef enum PortalStrategy
{
PORTAL_ONE_SELECT,
PORTAL_ONE_RETURNING,
PORTAL_ONE_MOD_WITH,
PORTAL_UTIL_SELECT,
PORTAL_MULTI_QUERY
} PortalStrategy;
typedef enum PortalStatus
{
PORTAL_NEW, /* freshly created */
PORTAL_DEFINED, /* PortalDefineQuery done */
PORTAL_READY, /* PortalStart complete, can run it */
PORTAL_ACTIVE, /* portal is running (can't delete it) */
PORTAL_DONE, /* portal is finished (don't re-run it) */
PORTAL_FAILED /* portal got error (can't re-run it) */
} PortalStatus;
typedef struct PortalData *Portal;
typedef struct PortalData
{
/* Bookkeeping data */
const char *name; /* portal's name */
const char *prepStmtName; /* source prepared statement (NULLif none) */
MemoryContext heap; /* subsidiary memory for portal */
ResourceOwner resowner; /* resources owned by portal */
void (*cleanup) (Portal portal); /* cleanup hook */
SubTransactionId createSubid; /* the ID of the creating subxact*/
/*
* ifcreateSubid is InvalidSubTransactionId, the portal is held over from
* aprevious transaction
*/
/* The query or queries the portal will execute */
const char *sourceText; /* text of query (as of 8.4, neverNULL) */
const char *commandTag; /* command tag for original query*/
List *stmts; /* PlannedStmts and/or utilitystatements */
CachedPlan *cplan; /* CachedPlan, if stmtsare from one */
ParamListInfo portalParams; /* paramsto pass to query*/
/* Features/options */
PortalStrategy strategy; /* see above */
int cursorOptions; /* DECLARE CURSOR option bits */
/* Status data */
PortalStatus status; /* see above */
bool portalPinned; /* a pinned portal can't bedropped */
/* If not NULL, Executor is active; call ExecutorEndeventually: */
QueryDesc *queryDesc; /* info needed for executorinvocation */
/* If portal returns tuples, this is their tupdesc:*/
TupleDesc tupDesc; /* descriptor for result tuples */
/* and these are the format codes to use for the columns:*/
int16 *formats; /* a format code for each column*/
/*
*Where we store tuples for a held cursor or a PORTAL_ONE_RETURNING or
*PORTAL_UTIL_SELECT query. (A cursor heldpast the end of its
*transaction no longer has any active executor state.)
*/
Tuplestorestate *holdStore; /* store for holdablecursors */
MemoryContext holdContext; /* memory containing holdStore */
/*
*atStart, atEnd and portalPos indicate the current cursor position.
*portalPos is zero before the first row, N after fetching N'th row of
*query. After we run off the end,portalPos = # of rows in query, and
*atEnd is true. If portalPos overflows,set posOverflow (this causes us
* tostop relying on its value for navigation). Note that atStart
*implies portalPos == 0, but not the reverse (portalPos could have
*overflowed).
*/
bool atStart;
bool atEnd;
bool posOverflow;
long portalPos;
/* Presentation data, primarily used by the pg_cursorssystem view */
TimestampTz creation_time; /* time at which this portal wasdefined */
bool visible; /* include this portal inpg_cursors? */
} PortalData;
执行器调用的主工作状态
typedef struct EState
{
NodeTag type;
/* Basic state for all query types: */
ScanDirection es_direction; /* current scan direction */
Snapshot es_snapshot; /* time qualto use */
Snapshot es_crosscheck_snapshot; /* crosschecktime qualforRI */
List *es_range_table; /* List of RangeTblEntry */
PlannedStmt *es_plannedstmt; /* link to top of plan tree */
JunkFilter *es_junkFilter; /* top-level junk filter, if any*/
/* If query can insert/delete tuples, the command ID tomark them with */
CommandId es_output_cid;
/* Info about target table(s) for insert/update/deletequeries: */
ResultRelInfo *es_result_relations; /* array ofResultRelInfos */
int es_num_result_relations; /* length of array */
ResultRelInfo *es_result_relation_info; /* currently active array elt*/
/* Stuff used for firing triggers: */
List *es_trig_target_relations; /* trigger-only ResultRelInfos */
TupleTableSlot *es_trig_tuple_slot; /* fortrigger output tuples */
TupleTableSlot *es_trig_oldtup_slot; /* for TriggerEnabled */
/* Parameter info: */
ParamListInfo es_param_list_info; /* values of external params*/
ParamExecData *es_param_exec_vals; /* values of internal params*/
/* Other working state: */
MemoryContext es_query_cxt; /* per-query context in whichEState lives */
List *es_tupleTable; /* List of TupleTableSlots */
List *es_rowMarks; /* List of ExecRowMarks */
uint32 es_processed; /* # of tuples processed */
Oid es_lastoid; /* last oidprocessed (byINSERT) */
int es_top_eflags; /* eflagspassed toExecutorStart */
int es_instrument; /* OR of InstrumentOption flags */
bool es_select_into; /* true if doing SELECT INTO */
bool es_into_oids; /* true to generate OIDs in SELECTINTO */
bool es_finished; /* true when ExecutorFinish isdone */
List *es_exprcontexts; /* List of ExprContexts withinEState */
List *es_subplanstates; /* List of PlanState for SubPlans*/
List *es_auxmodifytables; /* List of secondaryModifyTableStates */
/*
*this ExprContext is for per-output-tuple operations, such as constraint
*checks and index-value computations. Itwill be reset for each output
*tuple. Note that it will be created onlyif needed.
*/
ExprContext *es_per_tuple_exprcontext;
/*
*These fields are for re-evaluating plan qualswhen an updated tuple is
*substituted in READ COMMITTED mode. es_epqTuple[]contains tuples that
*scan plan nodes should return instead of whatever they'd normally
*return, or NULL if nothing to return; es_epqTupleSet[] is true if a
*particular array entry is valid; and es_epqScanDone[] is state to
*remember if the tuple has been returned already. Arrays are of size
*list_length(es_range_table) and are indexed by scan node scanrelid- 1.
*/
HeapTuple *es_epqTuple; /* array of EPQ substitute tuples*/
bool *es_epqTupleSet; /* true if EPQ tuple is provided */
bool *es_epqScanDone; /* true if EPQ tuple has been fetched */
/*
*this field added at end of structto avoid post-release ABI breakage in
*existing release branches. It'll be in amore logical place in 9.2.
*/
TupleTableSlot *es_trig_newtup_slot; /* for TriggerEnabled */
} EState;
下面是执行这个查询的portal相关内存结构图：
portal相关内存结构图
就到这儿吧。
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