本节简单介绍了PostgreSQL在执行逻辑优化中相关的数据结构,包括RangeSubselect/Alias/RangeVar/ResTarget/ColumnRef. ### 一、数据结构 **RangeSubselect** 出现在FROM语句中的子查询 ``` /* * RangeSubselect - subquery appearing in a FROM clause * 出现在FROM语句中的子查询 */ typedef struct RangeSubselect { NodeTag type; //是否有LATERAL? bool lateral; /* does it have LATERAL prefix? */ //未转换的子查询语句 Node *subquery; /* the untransformed sub-select clause */ //表别名和可选的列别名 Alias *alias; /* table alias & optional column aliases */ } RangeSubselect; ``` **Alias** 为RangeVar指定别名.别名可能同时重命名了表列. ``` /* * Alias - * specifies an alias for a range variable; the alias might also * specify renaming of columns within the table. * 为RangeVar指定别名.别名可能同时重命名了表列. * * Note: colnames is a list of Value nodes (always strings). In Alias structs * associated with RTEs, there may be entries corresponding to dropped * columns; these are normally empty strings (""). See parsenodes.h for info. * 注意:colnames是Value节点(通常是字符串)链表. * 在与RTEs相关的Alias结构体中,可能有跟已删除的列对应的条目. * 这些通常是空字符串("").详细可参考parsenodes.h */ typedef struct Alias { NodeTag type; //别名 char *aliasname; /* aliased rel name (never qualified) */ //列别名链表 List *colnames; /* optional list of column aliases */ } Alias; ``` **RangeVar** range variable,用于FROM语句中. ``` /* * RangeVar - range variable, used in FROM clauses * range variable,用于FROM语句中. * * Also used to represent table names in utility statements; there, the alias * field is not used, and inh tells whether to apply the operation * recursively to child tables. In some contexts it is also useful to carry * a TEMP table indication here. * 在工具语句中,用于表示表名,这时alias字段没有使用. * inh用于判断是否在子表中递归应用相关的操作. */ typedef struct RangeVar { NodeTag type; char *catalogname; /* the catalog (database) name, or NULL */ char *schemaname; /* the schema name, or NULL */ char *relname; /* the relation/sequence name */ bool inh; /* expand rel by inheritance? recursively act * on children? */ char relpersistence; /* see RELPERSISTENCE_* in pg_class.h */ Alias *alias; /* table alias & optional column aliases */ int location; /* token location, or -1 if unknown */ } RangeVar; ``` **ResTarget** 结果目标列(用于先前已转换的解析树的目标链表中) ``` /* * ResTarget - * result target (used in target list of pre-transformed parse trees) * 结果目标列(用于先前已转换的解析树的目标链表中) * * In a SELECT target list, 'name' is the column label from an * 'AS ColumnLabel' clause, or NULL if there was none, and 'val' is the * value expression itself. The 'indirection' field is not used. * 在SELECT的目标链表中, * 'name'是'AS ColumnLabel'语句中的列标签,如无则为NULL, * 'val'是value表达式本身. * 'indirection'未使用. * * INSERT uses ResTarget in its target-column-names list. Here, 'name' is * the name of the destination column, 'indirection' stores any subscripts * attached to the destination, and 'val' is not used. * INSERT在target-column-names链表中使用ResTarget. * 这里'name'是目标列名称,'indirection'存储了所有与目标相关的子脚本,'val'未使用. * * In an UPDATE target list, 'name' is the name of the destination column, * 'indirection' stores any subscripts attached to the destination, and * 'val' is the expression to assign. * 在UPDATE目标链表中,'name'是目标列名称,'indirection'存储了所有与目标相关的子脚本, * 'val'是与赋值相关的表达式. * * See A_Indirection for more info about what can appear in 'indirection'. * 详细参见A_Indirection('indirection') */ typedef struct ResTarget { NodeTag type; //列名或NULL char *name; /* column name or NULL */ //子脚本,字段名称,'*'或NIL List *indirection; /* subscripts, field names, and '*', or NIL */ //需要计算或赋值的值表达式 Node *val; /* the value expression to compute or assign */ //token的位置,-1表示未知 int location; /* token location, or -1 if unknown */ } ResTarget; ``` **ColumnRef** 指定对列或整个元组的引用 ``` /* * ColumnRef - specifies a reference to a column, or possibly a whole tuple * 指定对列或整个元组的引用 * * The "fields" list must be nonempty. It can contain string Value nodes * (representing names) and A_Star nodes (representing occurrence of a '*'). * Currently, A_Star must appear only as the last list element --- the grammar * is responsible for enforcing this! * "fields"链表不能为空.可能包含字符串Value节点和A_Star节点(表示出现了*). * 截止到目前为止,A_Star必须出现在最后一个链表元素中. * * Note: any container subscripting or selection of fields from composite columns * is represented by an A_Indirection node above the ColumnRef. However, * for simplicity in the normal case, initial field selection from a table * name is represented within ColumnRef and not by adding A_Indirection. */ typedef struct ColumnRef { NodeTag type; //字段名称(字符串值)链表或A_Star List *fields; /* field names (Value strings) or A_Star */ //token位置 int location; /* token location, or -1 if unknown */ } ColumnRef; ``` ### 二、源码解读 N/A ### 三、跟踪分析 **RangeSubselect/Alias** ``` (gdb) p *(Node *)($stmt->fromClause->head.data->ptr_value) $15 = {type = T_RangeSubselect} #实际类型是范围子查询RangeSubselect (gdb) set $fromclause=(RangeSubselect *)($stmt->fromClause->head.data->ptr_value) (gdb) p *$fromclause $16 = {type = T_RangeSubselect, lateral = false, subquery = 0x1666c18, alias = 0x1666d40} (gdb) p *($fromclause->subquery) #subquery,子查询是SelectStmt类型的节点 $17 = {type = T_SelectStmt} (gdb) p *($fromclause->alias) #alias,别名,实际值是字符串ret $18 = {type = T_Alias, aliasname = 0x1666d28 "ret", colnames = 0x0} ``` **RangeVar** ``` ... $43 = {type = T_RangeVar} (gdb) p *(RangeVar *)((JoinExpr *)($joinexpr->larg))->larg $44 = {type = T_RangeVar, catalogname = 0x0, schemaname = 0x0, relname = 0x1643380 "t_dwxx", inh = true, relpersistence = 112 'p', alias = 0x0, location = 82} ... ``` **ResTarget** ``` ... (gdb) p *(Node *)($subquerylarg->targetList->head.data->ptr_value) $26 = {type = T_ResTarget} (gdb) set $subvar=(ResTarget *)($subquerylarg->targetList->head.data->ptr_value) (gdb) p *$subvar {type = T_ResTarget, name = 0x0, indirection = 0x0, val = 0x1642c70, location = 23} ... ``` **ColumnRef** ``` ... (gdb) p *$restarget->val $25 = {type = T_ColumnRef} (gdb) p *(ColumnRef *)$restarget->val $26 = {type = T_ColumnRef, fields = 0x1a47a08, location = 7} (gdb) p *((ColumnRef *)$restarget->val)->fields $27 = {type = T_List, length = 2, head = 0x1a47a88, tail = 0x1a479e8} (gdb) p *(Node *)(((ColumnRef *)$restarget->val)->fields)->head.data->ptr_value $32 = {type = T_String} #fields链表的第1个元素是数据表,第2个元素是数据列 (gdb) p *(Value *)(((ColumnRef *)$restarget->val)->fields)->head.data->ptr_value $37 = {type = T_String, val = {ival = 27556248, str = 0x1a47998 "t_dwxx"}} (gdb) p *(Value *)(((ColumnRef *)$restarget->val)->fields)->tail.data->ptr_value $38 = {type = T_String, val = {ival = 27556272, str = 0x1a479b0 "dwmc"}} ... ``` ### 四、参考资料 N/A