主要介绍limit...offset...是如何实现的。首先看下数据结构。
1数据结构
Limit算子的描述结构式LimitState，它的结构如上图。PlanState是计划节点的描述信息；重要结构成员limitOffset和limitCount分别是limit算子计算offset和limit返回数量的表达式计算步骤，这个结构在ExecInitLimit中进行初始化；offset和count分别保存表达式计算的结果，也就是offset值和limit值；noCount表示是否有Limit，比如仅有offset语句；lstate表示算子执行的状态机；position作为中间使用值，表示最近一次返回tuple的位置；subSlot作为指针，指向子节点中获取的tuple。
2 执行
Limit算子的执行由函数ExecLimit来完成，看下它的代码：
主要分为2步：1）通过ExecLimit_guts函数来执行limit算子主要步骤，返回值result即为limit算子返回的tuple值；2）如果result为NULL也就是limit返回了所有tuple，并且设置了没有设置expect_rescan，就需要执行ExecSquelchNode通知下层节点，我已经获取了所有需要的tuple了，不再需要后续的tuple了。
我们主要看ExecLimit_guts函数。
ExecLimit_guts
主要是根据状态机状态来执行对应步骤。
typedef enum { LIMIT_INITIAL, /*LIMIT node初始状态 */ LIMIT_RESCAN, /* rescan after recomputing parameters */ LIMIT_EMPTY, /* 没有要返回的行了*/ LIMIT_INWINDOW, /* have returned a row in the window */ LIMIT_SUBPLANEOF, /* at EOF of subplan (within window) */ LIMIT_WINDOWEND, /* stepped off end of window */ LIMIT_WINDOWSTART /* stepped off beginning of window */ } LimitStateCond;
tatic TupleTableSlot * ExecLimit_guts(PlanState *pstate) { switch (node->lstate){ case LIMIT_INITIAL: //计算limit及offset recompute_limits(node); //计算完立即进入LIMIT_RESCAN case LIMIT_RESCAN://fetch tuple知道到达offset处 if (!ScanDirectionIsForward(direction)) return NULL; //limit窗口为0，返回null if (node->count <= 0 && !node->noCount){ node->lstate = LIMIT_EMPTY; return NULL; } for (;;){ slot = ExecProcNode(outerPlan); if (TupIsNull(slot)){ //子节点没有slot了 node->lstate = LIMIT_EMPTY; return NULL; } node->subSlot = slot;//下层节点获取后存入这里 if (++node->position > node->offset) break;//满足offset窗口 } //获取了满足窗口的第一个tuple node->lstate = LIMIT_INWINDOW; break; case LIMIT_EMPTY: return NULL; case LIMIT_INWINDOW: if (ScanDirectionIsForward(direction)){ if (!node->noCount && node->position - node->offset >= node->count){ node->lstate = LIMIT_WINDOWEND;//达到了limit个数 if (!(node->ps.state->es_top_eflags & EXEC_FLAG_BACKWARD)) (void) ExecShutdownNode(outerPlan); return NULL; }//fetch一个tuple slot = ExecProcNode(outerPlan); if (TupIsNull(slot)){ node->lstate = LIMIT_SUBPLANEOF; return NULL; } node->subSlot = slot; node->position++; }else{ if (node->position <= node->offset + 1){ node->lstate = LIMIT_WINDOWSTART; return NULL; } slot = ExecProcNode(outerPlan); if (TupIsNull(slot)) elog(ERROR, "LIMIT subplan failed to run backwards"); node->subSlot = slot; node->position--; } break; case LIMIT_SUBPLANEOF: if (ScanDirectionIsForward(direction)) return NULL; slot = ExecProcNode(outerPlan); if (TupIsNull(slot)) elog(ERROR, "LIMIT subplan failed to run backwards"); node->subSlot = slot; node->lstate = LIMIT_INWINDOW; /* position does not change 'cause we didn't advance it before */ break; case LIMIT_WINDOWEND: if (ScanDirectionIsForward(direction)) return NULL; slot = node->subSlot; node->lstate = LIMIT_INWINDOW; /* position does not change 'cause we didn't advance it before */ break; case LIMIT_WINDOWSTART: if (!ScanDirectionIsForward(direction)) return NULL; slot = node->subSlot; node->lstate = LIMIT_INWINDOW; break; default: elog(ERROR, "impossible LIMIT state: %d", (int) node->lstate); slot = NULL; /* keep compiler quiet */ break; } return slot; }
计算处limit值和offset后，首先进入LIMIT_RESCAN状态，不断从下层节点fetch tuple直到到达offset处，然后进入LIMIT_INWINDOW状态：获取一个记录并返回，直到达到limit个数。
