PostgreSQL 源码解读（81）- 查询语句#66（Rev

本节Review exec_simple_query函数的实现逻辑，该函数响应并处理"Simple Query"协议的消息请求。

一、源码解读

exec_simple_query函数由PostgresMain函数调用,其调用栈如下:

#3  0x00000000008c5c35 in exec_simple_query (query_string=0x2eed1a8 "select * from t1;") at postgres.c:1050
#4  0x00000000008ca0f8 in PostgresMain (argc=1, argv=0x2f16cb8, dbname=0x2f16b20 "testdb", username=0x2ee9d48 "xdb")
    at postgres.c:4159
#5  0x0000000000825880 in BackendRun (port=0x2f0eb00) at postmaster.c:4361
#6  0x0000000000824fe4 in BackendStartup (port=0x2f0eb00) at postmaster.c:4033
#7  0x0000000000821371 in ServerLoop () at postmaster.c:1706
#8  0x0000000000820c09 in PostmasterMain (argc=1, argv=0x2ee7d00) at postmaster.c:1379
#9  0x0000000000747ea7 in main (argc=1, argv=0x2ee7d00) at main.c:228

该函数的实现逻辑详见代码注释.

/*
 * exec_simple_query
 *
 * Execute a "simple Query" protocol message.
 * 响应并执行"simple Query"协议消息请求
 */
/*
输入：
    query_string-SQL语句
输出：
    无
*/
static void
exec_simple_query(const char *query_string)
{
    CommandDest dest = whereToSendOutput;//输出到哪里的定义
    MemoryContext oldcontext;//存储原内存上下文
    List       *parsetree_list;//分析树列表
    ListCell   *parsetree_item;//分析树中的ITEM
    bool        save_log_statement_stats = log_statement_stats;//是否保存统计信息，false
    bool        was_logged = false;//Log？
    bool        use_implicit_block;//是否使用隐式事务块
    char        msec_str[32];

    /*
     * Report query to various monitoring facilities.
     * 监控信息
     */
    debug_query_string = query_string;

    pgstat_report_activity(STATE_RUNNING, query_string);//统计信息

    TRACE_POSTGRESQL_QUERY_START(query_string);

    /*
     * We use save_log_statement_stats so ShowUsage doesn't report incorrect
     * results because ResetUsage wasn't called.
     * 如save_log_statement_stats为T,则调用ResetUsage函数
     */
    if (save_log_statement_stats)
        ResetUsage();

    /*
     * Start up a transaction command.  All queries generated by the
     * query_string will be in this same command block, *unless* we find a
     * BEGIN/COMMIT/ABORT statement; we have to force a new xact command after
     * one of those, else bad things will happen in xact.c. (Note that this
     * will normally change current memory context.)
     * 启动一个事务命令。
     * 由query_string生成的所有查询都将位于同一个命令块中，*除非*找到BEGIN/COMMIT/ABORT语句;
     * 必须在其中一个命令之后强制执行新的xact命令，否则xact.c中会发生意想不到事情。
     * (注意，这通常会改变当前内存上下文。)
     */
    start_xact_command();//启动事务

    /*
     * Zap any pre-existing unnamed statement.  (While not strictly necessary,
     * it seems best to define simple-Query mode as if it used the unnamed
     * statement and portal; this ensures we recover any storage used by prior
     * unnamed operations.)
     * 删除任何预先存在的未命名语句。
     * (虽然不是严格必要的，但最好定义简单查询模式，就像使用未命名语句和门户接口一样;
     * 这确保我们恢复以前未命名操作所使用的存储信息。)
     */
    drop_unnamed_stmt();//清除未命名语句

    /*
     * Switch to appropriate context for constructing parsetrees.
     * 切换至合适的内存上下文
     */
    oldcontext = MemoryContextSwitchTo(MessageContext);//切换内存上下文

    /*
     * Do basic parsing of the query or queries (this should be safe even if
     * we are in aborted transaction state!)
     * 执行查询(或多个查询)的基本解析
     * (即使我们处于中止的事务状态，这也应该是安全的!)
     */
    parsetree_list = pg_parse_query(query_string);//解析输入的查询语句，获得解析树List（元素是RawStmt nodes）

    /* Log immediately if dictated by log_statement */
    //如需要记录日志,则Log这些信息
    if (check_log_statement(parsetree_list))//日志记录
    {
        ereport(LOG,
                (errmsg("statement: %s", query_string),
                 errhidestmt(true),
                 errdetail_execute(parsetree_list)));
        was_logged = true;
    }

    /*
     * Switch back to transaction context to enter the loop.
     * 切换回去原事务上下文
     */
    MemoryContextSwitchTo(oldcontext);//切换回原内存上下文

    /*
     * For historical reasons, if multiple SQL statements are given in a
     * single "simple Query" message, we execute them as a single transaction,
     * unless explicit transaction control commands are included to make
     * portions of the list be separate transactions.  To represent this
     * behavior properly in the transaction machinery, we use an "implicit"
     * transaction block.
     * 由于历史原因，如果在单个“简单查询”消息中给出了多个SQL语句，那么我们将作为单个事务执行它们，
     * 除非包含显式事务控制命令，使链表中的部分成为单独的事务。
     * 为了在事务机制中正确地表示这种行为，使用了一个“隐式”事务块。
     */
    //如果分析树条目>1，使用隐式事务块（多条SQL语句在同一个事务中）
    use_implicit_block = (list_length(parsetree_list) > 1);

    /*
     * Run through the raw parsetree(s) and process each one.
     * 对分析树中的每一个条目进行处理
     */
    foreach(parsetree_item, parsetree_list)//
    {
        RawStmt    *parsetree = lfirst_node(RawStmt, parsetree_item);//分析树List中的元素为RawStmt指针类型
        bool        snapshot_set = false;//是否设置快照？
        const char *commandTag;//命令标识
        char        completionTag[COMPLETION_TAG_BUFSIZE];//完成标记，如INSERT 0 1之类的字符串
        List       *querytree_list,//查询树List
                   *plantree_list;//执行计划List
        Portal      portal;//“门户”变量
        DestReceiver *receiver;//目标接收端
        int16       format;//

        /*
         * Get the command name for use in status display (it also becomes the
         * default completion tag, down inside PortalRun).  Set ps_status and
         * do any special start-of-SQL-command processing needed by the
         * destination.
         * 获取用于状态显示的命令名称(在PortalRun内部，它也成为默认的完成标记)。
         * 设置ps_status并执行目标语句所需要的start-of-SQL-command处理。
         */
        commandTag = CreateCommandTag(parsetree->stmt);//创建命令标记，插入数据则为INSERT

        set_ps_display(commandTag, false);

        BeginCommand(commandTag, dest);//do Nothing!

        /*
         * If we are in an aborted transaction, reject all commands except
         * COMMIT/ABORT.  It is important that this test occur before we try
         * to do parse analysis, rewrite, or planning, since all those phases
         * try to do database accesses, which may fail in abort state. (It
         * might be safe to allow some additional utility commands in this
         * state, but not many...)
         * 如果我们处于事务中止状态中，拒绝除COMMIT/ABORT之外的所有命令。
         * 在尝试进行解析分析、重写或计划之前进行此测试是很重要的，
         * 因为所有这些阶段都尝试进行数据库访问，而在abort状态下可能会失败。
         * (在这种状态下允许一些额外的实用程序命令可能是安全的，但不是很多……)
         */
        if (IsAbortedTransactionBlockState() &&
            !IsTransactionExitStmt(parsetree->stmt))
            ereport(ERROR,
                    (errcode(ERRCODE_IN_FAILED_SQL_TRANSACTION),
                     errmsg("current transaction is aborted, "
                            "commands ignored until end of transaction block"),
                     errdetail_abort()));

        /* Make sure we are in a transaction command */
        start_xact_command();//确认在事务中

        /*
         * If using an implicit transaction block, and we're not already in a
         * transaction block, start an implicit block to force this statement
         * to be grouped together with any following ones.  (We must do this
         * each time through the loop; otherwise, a COMMIT/ROLLBACK in the
         * list would cause later statements to not be grouped.)
         * 如果使用隐式事务块(还没有使用atransaction块)，启动一个隐式事务块来强制将该语句与以下语句组合在一起。
         * (我们每次通过循环时都必须这样做;否则，链表中的提交/回滚将导致后面的语句没有分组。
         */
        if (use_implicit_block)
            BeginImplicitTransactionBlock();//隐式事务，进入事务块

        /* If we got a cancel signal in parsing or prior command, quit */
        //如果在解析或者解析之前接收到取消的信号,则退出
        CHECK_FOR_INTERRUPTS();

        /*
         * Set up a snapshot if parse analysis/planning will need one.
         * 如果解析/分析/计划过程需要snapshot,则创建一个
         */
        if (analyze_requires_snapshot(parsetree))//是否需要快照进行分析？增删改查均需要
        {
            PushActiveSnapshot(GetTransactionSnapshot());//活动快照入栈
            snapshot_set = true;
        }

        /*
         * OK to analyze, rewrite, and plan this query.
         * 准备工作妥当,可以进行分析/重写和计划查询语句了
         *
         * Switch to appropriate context for constructing querytrees (again,
         * these must outlive the execution context).
         * 切换至合适的内存上下文中
         */
        oldcontext = MemoryContextSwitchTo(MessageContext);//切换内存上下文
        //根据分析树获得查询树，返回List（元素为Query）
        querytree_list = pg_analyze_and_rewrite(parsetree, query_string,
                                                NULL, 0, NULL);
        //根据查询树获取计划树，返回List（元素为PlannedStmt）
        plantree_list = pg_plan_queries(querytree_list,
                                        CURSOR_OPT_PARALLEL_OK, NULL);

        /* Done with the snapshot used for parsing/planning */
        //启用了快照,则出栈
        if (snapshot_set)
            PopActiveSnapshot();//

        /* If we got a cancel signal in analysis or planning, quit */
        //如接收到取消信号,则退出
        CHECK_FOR_INTERRUPTS();

        /*
         * Create unnamed portal to run the query or queries in. If there
         * already is one, silently drop it.
         * 创建匿名的门户接口来执行查询,如Portal已存在,则先drop it
         */
        portal = CreatePortal("", true, true);//创建匿名Portal变量
        /* Don't display the portal in pg_cursors */
        //该portal不在pg_cursors中出现
        portal->visible = false;

        /*
         * We don't have to copy anything into the portal, because everything
         * we are passing here is in MessageContext, which will outlive the
         * portal anyway.
         * 不需要将任何内容复制到Portal中，
         * 因为在这里传递的所有内容都在MessageContext中，它的生命周期将比Portal更长。
         */
        PortalDefineQuery(portal,
                          NULL,
                          query_string,
                          commandTag,
                          plantree_list,
                          NULL);//给Portal变量赋值

        /*
         * Start the portal.  No parameters here.
         * 启动Portal,不需要参数
         */
        PortalStart(portal, NULL, 0, InvalidSnapshot);//为PortalRun作准备

        /*
         * Select the appropriate output format: text unless we are doing a
         * FETCH from a binary cursor.  (Pretty grotty to have to do this here
         * --- but it avoids grottiness in other places.  Ah, the joys of
         * backward compatibility...)
         * 选择适当的输出格式:文本，除了我们正在从二进制游标进行提取。
         * (不得不在这里这样做实在是太糟糕了——但在其他地方却避免了这种情况。啊，向后兼容的乐趣……
         */
        format = 0;             /* 默认为TEXT;TEXT is default */
        if (IsA(parsetree->stmt, FetchStmt))
        {
            FetchStmt  *stmt = (FetchStmt *) parsetree->stmt;

            if (!stmt->ismove)
            {
                Portal      fportal = GetPortalByName(stmt->portalname);

                if (PortalIsValid(fportal) &&
                    (fportal->cursorOptions & CURSOR_OPT_BINARY))
                    format = 1; /* 二进制格式;BINARY */
            }
        }
        PortalSetResultFormat(portal, 1, &format);//设置结果返回的格式，默认为TEXT

        /*
         * Now we can create the destination receiver object.
         * 现在可以创建目标接收器对象了
         */
        //创建目标接收器(如使用psql则为：printtup DestReceiver)
        receiver = CreateDestReceiver(dest);
        if (dest == DestRemote)
            SetRemoteDestReceiverParams(receiver, portal);

        /*
         * Switch back to transaction context for execution.
         * 切换回原内存上下文
         */
        MemoryContextSwitchTo(oldcontext);//

        /*
         * Run the portal to completion, and then drop it (and the receiver).
         * 执行PortalRun,然后清除
         */
        (void) PortalRun(portal,
                         FETCH_ALL,
                         true,  /* always top level */
                         true,
                         receiver,
                         receiver,
                         completionTag);//执行
        //执行完毕，销毁接收器
        receiver->rDestroy(receiver);
        //清除Portal中的资源
        PortalDrop(portal, false);

        if (lnext(parsetree_item) == NULL)//所有语句已执行完毕
        {
            /*
             * If this is the last parsetree of the query string, close down
             * transaction statement before reporting command-complete.  This
             * is so that any end-of-transaction errors are reported before
             * the command-complete message is issued, to avoid confusing
             * clients who will expect either a command-complete message or an
             * error, not one and then the other.  Also, if we're using an
             * implicit transaction block, we must close that out first.
             * 如果这是查询字符串的最后一个parsetree，在报告命令完成之前关闭事务语句。
             * 这样，在发出命令完整的消息之前就会报告任何事务结束错误，以避免混淆视听，
             * 因为客户端希望看到命令完整的消息或错误，而不是一个错误，然后是另一个错误。
             * 另外，如果我们使用隐式事务块，我们必须首先关闭它。
             */
            if (use_implicit_block)
                EndImplicitTransactionBlock();//结束事务
            finish_xact_command();//结束事务
        }
        else if (IsA(parsetree->stmt, TransactionStmt))//事务语句？BEGIN/COMMIT/ABORT...
        {
            /*
             * If this was a transaction control statement, commit it. We will
             * start a new xact command for the next command.
             * 如果这是一个事务控制语句，提交此语句。
             * 我们将为下一个命令启动一个新的xact命令。
             */
            finish_xact_command();
        }
        else
        {
            /*
             * We need a CommandCounterIncrement after every query, except
             * those that start or end a transaction block.
             * 在每次查询之后，我们都需要一个CommandCounterIncrement，除了那些启动或结束事务块的查询。
             */
            CommandCounterIncrement();//命令+1（对应Tuple中的cid）
        }

        /*
         * Tell client that we're done with this query.  Note we emit exactly
         * one EndCommand report for each raw parsetree, thus one for each SQL
         * command the client sent, regardless of rewriting. (But a command
         * aborted by error will not send an EndCommand report at all.)
         * 告诉客户端已经完成了这个查询。注意，对于每个原始的parsetree，只发出一个EndCommand报告，
         * 因此，对于客户机发送的每个SQL命令，只发出一个EndCommand报告，而不考虑重写。
         * (注:由于错误而中止的命令根本不会发送EndCommand报告。)
         */
        EndCommand(completionTag, dest);//命令Done
    }                           /* end loop over parsetrees */
    
    //所有语句结束
    /*
     * Close down transaction statement, if one is open.  (This will only do
     * something if the parsetree list was empty; otherwise the last loop
     * iteration already did it.)
     * 如果事务是打开的，则关闭。
     * (只有当parsetree链表为空时，才会执行某些操作;否则，最后一次循环迭代已经完成了。
     */
    finish_xact_command();

    /*
     * If there were no parsetrees, return EmptyQueryResponse message.
     * 如果parsetrees链表已清空,返回EmptyQueryResponse消息
     */
    if (!parsetree_list)
        NullCommand(dest);

    /*
     * Emit duration logging if appropriate.
     * 如需要记录日志,则在合适的时候记录
     */
    switch (check_log_duration(msec_str, was_logged))
    {
        case 1:
            ereport(LOG,
                    (errmsg("duration: %s ms", msec_str),
                     errhidestmt(true)));
            break;
        case 2:
            ereport(LOG,
                    (errmsg("duration: %s ms  statement: %s",
                            msec_str, query_string),
                     errhidestmt(true),
                     errdetail_execute(parsetree_list)));
            break;
    }

    if (save_log_statement_stats)
        ShowUsage("QUERY STATISTICS");

    TRACE_POSTGRESQL_QUERY_DONE(query_string);

    debug_query_string = NULL;
}

二、参考资料

PG Document:Query Planning