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;
...
}
}