PostgreSQL服务过程中的那些事二:Pg服务进程处理简单查询六:执
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。 typedefenum PortalStrategy { PORTAL_ONE_SELECT, PORTAL_ONE_RETURNING, PORTAL_ONE_MOD_WITH, PORTAL_UTIL_SELECT, PORTAL_MULTI_QUERY } PortalStrategy;
typedefenum 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;
typedefstruct PortalData *Portal;
typedefstruct PortalData { /* Bookkeeping data */ constchar *name; /* portal's name */ constchar *prepStmtName; /* source prepared statement (NULLif none) */ MemoryContextheap; /* subsidiary memory for portal */ ResourceOwnerresowner; /* resources owned by portal */ void (*cleanup) (Portal portal); /* cleanup hook */ SubTransactionIdcreateSubid; /* 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 */ constchar *sourceText; /* text of query (as of 8.4,neverNULL) */ constchar *commandTag; /* command tag for original query*/ List *stmts; /* PlannedStmts and/or utilitystatements */ CachedPlan *cplan; /* CachedPlan,if stmts are from one */
ParamListInfoportalParams; /* params to pass to query*/
/* Features/options */ PortalStrategystrategy; /* see above */ int cursorOptions; /* DECLARE CURSOR option bits */
/* Status data */ PortalStatusstatus; /* 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 */ MemoryContextholdContext; /* 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 */ TimestampTzcreation_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 typedefstruct EState { NodeTag type;
/* Basic state for all query types: */ ScanDirectiones_direction; /* current scan direction */ Snapshot es_snapshot; /* time qual to use */ Snapshot es_crosscheck_snapshot; /* crosscheck time qual forRI */ 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: */ ParamListInfoes_param_list_info; /* values of external params */ ParamExecData *es_param_exec_vals; /* values of internal params */
/* Other working state: */ MemoryContextes_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 oid processed (byINSERT) */
int es_top_eflags; /* eflags passed 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 quals when 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 struct to 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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