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PostgreSQL 源码解读(174)- 内核研发#2(如何添加系统字段)#2

原创 PostgreSQL 作者:husthxd 时间:2019-04-19 17:20:40 0 删除 编辑

本节继续处理上一节给出的任务,重点介绍第二部分:输出,实现结果投影列名称命名为sysdate

一、输出

参照current_date函数,首先通过日志跟踪其parse tree,重点关注targetList


 2019-04-19 17:06:15.591 CST,"xdb","testdb",1693,"[local]",5cb98b16.69d,3,"SELECT",2019-04-19 16:47:18 CST,3/4,0,LOG,00000,"parse tree:","   {QUERY 
   :commandType 1 
   ...
   :targetList (
      {TARGETENTRY 
      :expr 
         {SQLVALUEFUNCTION 
         :op 0 
         :type 1082 
         :typmod -1 
         :location 7
         }
      :resno 1 
      :resname current_date 
      :ressortgroupref 0 
      :resorigtbl 0 
      :resorigcol 0 
      :resjunk false
      }
   )
   ...
   :stmt_len 27
   }

分析源代码,找到与输出相关的文件是src/backend/parser/parse_target.c,相关的函数为transformTargetEntry,跟踪此函数,打印调用栈:


(gdb) bt
#0  transformTargetEntry (pstate=0x16afae8, node=0x16af770, expr=0x0, exprKind=EXPR_KIND_SELECT_TARGET, colname=0x0, 
    resjunk=false) at parse_target.c:93
#1  0x000000000060b1f9 in transformTargetList (pstate=0x16afae8, targetlist=0x16af828, exprKind=EXPR_KIND_SELECT_TARGET)
    at parse_target.c:191
#2  0x00000000005b1e3a in transformSelectStmt (pstate=0x16afae8, stmt=0x16af938) at analyze.c:1243
#3  0x00000000005b03db in transformStmt (pstate=0x16afae8, parseTree=0x16af938) at analyze.c:301
#4  0x00000000005b02b6 in transformOptionalSelectInto (pstate=0x16afae8, parseTree=0x16af938) at analyze.c:246
#5  0x00000000005b0174 in transformTopLevelStmt (pstate=0x16afae8, parseTree=0x16afa50) at analyze.c:196
#6  0x00000000005affcc in parse_analyze (parseTree=0x16afa50, sourceText=0x16aed78 "select current_date from t1;", 
    paramTypes=0x0, numParams=0, queryEnv=0x0) at analyze.c:116
#7  0x00000000008bb78c in pg_analyze_and_rewrite (parsetree=0x16afa50, 
    query_string=0x16aed78 "select current_date from t1;", paramTypes=0x0, numParams=0, queryEnv=0x0) at postgres.c:689
#8  0x00000000008bbddd in exec_simple_query (query_string=0x16aed78 "select current_date from t1;") at postgres.c:1070
#9  0x00000000008c01f3 in PostgresMain (argc=1, argv=0x16dcd28, dbname=0x16dcb90 "testdb", username=0x16aba98 "xdb")
    at postgres.c:4182
#10 0x000000000081e0ce in BackendRun (port=0x16d0b50) at postmaster.c:4361
#11 0x000000000081d841 in BackendStartup (port=0x16d0b50) at postmaster.c:4033
#12 0x0000000000819c3b in ServerLoop () at postmaster.c:1706
#13 0x00000000008194f1 in PostmasterMain (argc=1, argv=0x16a9a50) at postmaster.c:1379
#14 0x0000000000742993 in main (argc=1, argv=0x16a9a50) at main.c:228

其中FigureColnameInternal函数为current_date设置输出列名,添加以下代码,则实现输出sysdate:


//Hacker : 添加系统列
case SVFOP_ZZ_SYSDATE:
                    *name = "sysdate";//zz_sysdate -> sysdate
                    return 2;

相关源码解读


/*
 * transformTargetEntry()
 *    Transform any ordinary "expression-type" node into a targetlist entry.
 *    This is exported so that parse_clause.c can generate targetlist entries
 *    for ORDER/GROUP BY items that are not already in the targetlist.
 *    转换所有普通的"表达式类型expression-type"节点为tagetlist条目.
 *    TargetEntry作为输出参数以便parse_clause.c
 *    可以为ORDER/GROUP BY等未在targetlist中的items生成targetlist条目
 *
 * node        the (untransformed) parse tree for the value expression.
 * node     未经变换的值表达式
 *
 * expr        the transformed expression, or NULL if caller didn't do it yet.
 * expr     已转换的表达式,如调用者未转换则为NULL
 *
 * exprKind expression kind (EXPR_KIND_SELECT_TARGET, etc)
 * exprKind 表达式类型(如EXPR_KIND_SELECT_TARGET等)
 *
 * colname    the column name to be assigned, or NULL if none yet set.
 * colname  分配的列名或者为NULL
 *
 * resjunk    true if the target should be marked resjunk, ie, it is not
 *            wanted in the final projected tuple.
 * resjunk  如目标应标记为resjunk,则为T,比如该列不希望投影为最终的元组
 */
TargetEntry *
transformTargetEntry(ParseState *pstate,
                     Node *node,
                     Node *expr,
                     ParseExprKind exprKind,
                     char *colname,
                     bool resjunk)
{
    /* Transform the node if caller didn't do it already */
    //expr为NULL,则转换之
    if (expr == NULL)
    {
        /*
         * If it's a SetToDefault node and we should allow that, pass it
         * through unmodified.  (transformExpr will throw the appropriate
         * error if we're disallowing it.)
         */
        if (exprKind == EXPR_KIND_UPDATE_SOURCE && IsA(node, SetToDefault))
            expr = node;
        else
            expr = transformExpr(pstate, node, exprKind);
    }
    if (colname == NULL && !resjunk)
    {
        /*
         * Generate a suitable column name for a column without any explicit
         * 'AS ColumnName' clause.
         * 如非显式指定列名(AS ColumnName),产生一个合适的列名
         */
        colname = FigureColname(node);
    }
    //返回TargetEntry
    return makeTargetEntry((Expr *) expr,
                           (AttrNumber) pstate->p_next_resno++,
                           colname,
                           resjunk);
}
/*
 * FigureColname -
 *      if the name of the resulting column is not specified in the target
 *      list, we have to guess a suitable name.  The SQL spec provides some
 *      guidance, but not much...
 *
 * Note that the argument is the *untransformed* parse tree for the target
 * item.  This is a shade easier to work with than the transformed tree.
 */
char *
FigureColname(Node *node)
{
    char       *name = NULL;
    (void) FigureColnameInternal(node, &name);
    if (name != NULL)
        return name;
    /* default result if we can't guess anything */
    return "?column?";
}
/*
 * FigureColnameInternal -
 *      internal workhorse for FigureColname
 *
 * Return value indicates strength of confidence in result:
 *        0 - no information
 *        1 - second-best name choice
 *        2 - good name choice
 * The return value is actually only used internally.
 * If the result isn't zero, *name is set to the chosen name.
 */
static int
FigureColnameInternal(Node *node, char **name)
{
    int            strength = 0;
    if (node == NULL)
        return strength;
    switch (nodeTag(node))
    {
        ...
        case T_SQLValueFunction:
            /* make these act like a function or variable */
            switch (((SQLValueFunction *) node)->op)
            {
                case SVFOP_CURRENT_DATE:
                    *name = "current_date";
                    return 2;
                ...
                //Hacker : 添加系统列
                case SVFOP_ZZ_SYSDATE:
                    *name = "sysdate";//zz_sysdate -> sysdate
                    return 2;
            }
            break;
        ...
    }
}

二、参考资料

Flex&Bison

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/6906/viewspace-2641981/,如需转载,请注明出处,否则将追究法律责任。

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