PostgreSQL在何处处理 sql查询之五十七
接着追踪 ->pathlist：
初始化可能是在这里完成的？
/* * add_path * Consider a potential implementation path for the specified parent rel, * and add it to the rel's pathlist if it is worthy of consideration. * A path is worthy if it has a better sort order (better pathkeys) or * cheaper cost (on either dimension), or generates fewer rows, than any * existing path that has the same or superset parameterization rels. * * We also remove from the rel's pathlist any old paths that are dominated * by new_path --- that is, new_path is cheaper, at least as well ordered, * generates no more rows, and requires no outer rels not required by the * old path. * * In most cases, a path with a superset parameterization will generate * fewer rows (since it has more join clauses to apply), so that those two * figures of merit move in opposite directions; this means that a path of * one parameterization can seldom dominate a path of another. But such * cases do arise, so we make the full set of checks anyway. * * There is one policy decision embedded in this function, along with its * sibling add_path_precheck: we treat all parameterized paths as having * NIL pathkeys, so that they compete only on cost. This is to reduce * the number of parameterized paths that are kept. See discussion in * src/backend/optimizer/README. * * The pathlist is kept sorted by total_cost, with cheaper paths * at the front. Within this routine, that's simply a speed hack: * doing it that way makes it more likely that we will reject an inferior * path after a few comparisons, rather than many comparisons. * However, add_path_precheck relies on this ordering to exit early * when possible. * * NOTE: discarded Path objects are immediately pfree'd to reduce planner * memory consumption. We dare not try to free the substructure of a Path, * since much of it may be shared with other Paths or the query tree itself; * but just recycling discarded Path nodes is a very useful savings in * a large join tree. We can recycle the List nodes of pathlist, too. * * BUT: we do not pfree IndexPath objects, since they may be referenced as * children of BitmapHeapPaths as well as being paths in their own right. * * 'parent_rel' is the relation entry to which the path corresponds. * 'new_path' is a potential path for parent_rel. * * Returns nothing, but modifies parent_rel->pathlist. */ void add_path(RelOptInfo *parent_rel, Path *new_path) { bool accept_new = true; /* unless we find a superior old path */ ListCell *insert_after = NULL; /* where to insert new item */ List *new_path_pathkeys; ListCell *p1; ListCell *p1_prev; ListCell *p1_next; /* * This is a convenient place to check for query cancel --- no part of the * planner goes very long without calling add_path(). */ CHECK_FOR_INTERRUPTS(); /* Pretend parameterized paths have no pathkeys, per comment above */ new_path_pathkeys = new_path->param_info ? NIL : new_path->pathkeys; /* * Loop to check proposed new path against old paths. Note it is possible * for more than one old path to be tossed out because new_path dominates * it. * * We can't use foreach here because the loop body may delete the current * list cell. */ p1_prev = NULL; for (p1 = list_head(parent_rel->pathlist); p1 != NULL; p1 = p1_next) { Path *old_path = (Path *) lfirst(p1); bool remove_old = false; /* unless new proves superior */ PathCostComparison costcmp; PathKeysComparison keyscmp; BMS_Comparison outercmp; p1_next = lnext(p1); /* * Do a fuzzy cost comparison with 1% fuzziness limit. (XXX does this * percentage need to be user-configurable?) */ costcmp = compare_path_costs_fuzzily(new_path, old_path, 1.01); /* * If the two paths compare differently for startup and total cost, * then we want to keep both, and we can skip comparing pathkeys and * required_outer rels. If they compare the same, proceed with the * other comparisons. Row count is checked last. (We make the tests * in this order because the cost comparison is most likely to turn * out "different", and the pathkeys comparison next most likely. As * explained above, row count very seldom makes a difference, so even * though it's cheap to compare there's not much point in checking it * earlier.) */ if (costcmp != COSTS_DIFFERENT) { /* Similarly check to see if either dominates on pathkeys */ List *old_path_pathkeys; old_path_pathkeys = old_path->param_info ? NIL : old_path->pathkeys; keyscmp = compare_pathkeys(new_path_pathkeys, old_path_pathkeys); if (keyscmp != PATHKEYS_DIFFERENT) { switch (costcmp) { case COSTS_EQUAL: outercmp = bms_subset_compare(PATH_REQ_OUTER(new_path), PATH_REQ_OUTER(old_path)); if (keyscmp == PATHKEYS_BETTER1) { if ((outercmp == BMS_EQUAL || outercmp == BMS_SUBSET1) && new_path->rows <= old_path->rows) remove_old = true; /* new dominates old */ } else if (keyscmp == PATHKEYS_BETTER2) { if ((outercmp == BMS_EQUAL || outercmp == BMS_SUBSET2) && new_path->rows >= old_path->rows) accept_new = false; /* old dominates new */ } else /* keyscmp == PATHKEYS_EQUAL */ { if (outercmp == BMS_EQUAL) { /* * Same pathkeys and outer rels, and fuzzily * the same cost, so keep just one; to decide * which, first check rows and then do a fuzzy * cost comparison with very small fuzz limit. * (We used to do an exact cost comparison, * but that results in annoying * platform-specific plan variations due to * roundoff in the cost estimates.) If things * are still tied, arbitrarily keep only the * old path. Notice that we will keep only * the old path even if the less-fuzzy * comparison decides the startup and total * costs compare differently. */ if (new_path->rows < old_path->rows) remove_old = true; /* new dominates old */ else if (new_path->rows > old_path->rows) accept_new = false; /* old dominates new */ else if (compare_path_costs_fuzzily(new_path, old_path, 1.0000000001) == COSTS_BETTER1) remove_old = true; /* new dominates old */ else accept_new = false; /* old equals or * dominates new */ } else if (outercmp == BMS_SUBSET1 && new_path->rows <= old_path->rows) remove_old = true; /* new dominates old */ else if (outercmp == BMS_SUBSET2 && new_path->rows >= old_path->rows) accept_new = false; /* old dominates new */ /* else different parameterizations, keep both */ } break; case COSTS_BETTER1: if (keyscmp != PATHKEYS_BETTER2) { outercmp = bms_subset_compare(PATH_REQ_OUTER(new_path), PATH_REQ_OUTER(old_path)); if ((outercmp == BMS_EQUAL || outercmp == BMS_SUBSET1) && new_path->rows <= old_path->rows) remove_old = true; /* new dominates old */ } break; case COSTS_BETTER2: if (keyscmp != PATHKEYS_BETTER1) { outercmp = bms_subset_compare(PATH_REQ_OUTER(new_path), PATH_REQ_OUTER(old_path)); if ((outercmp == BMS_EQUAL || outercmp == BMS_SUBSET2) && new_path->rows >= old_path->rows) accept_new = false; /* old dominates new */ } break; case COSTS_DIFFERENT: /* * can't get here, but keep this case to keep compiler * quiet */ break; } } } /* * Remove current element from pathlist if dominated by new. */ if (remove_old) { parent_rel->pathlist = list_delete_cell(parent_rel->pathlist, p1, p1_prev); /* * Delete the data pointed-to by the deleted cell, if possible */ if (!IsA(old_path, IndexPath)) pfree(old_path); /* p1_prev does not advance */ } else { /* new belongs after this old path if it has cost >= old's */ if (new_path->total_cost >= old_path->total_cost) insert_after = p1; /* p1_prev advances */ p1_prev = p1; } /* * If we found an old path that dominates new_path, we can quit * scanning the pathlist; we will not add new_path, and we assume * new_path cannot dominate any other elements of the pathlist. */ if (!accept_new) break; } if (accept_new) { /* Accept the new path: insert it at proper place in pathlist */ if (insert_after) lappend_cell(parent_rel->pathlist, insert_after, new_path); else parent_rel->pathlist = lcons(new_path, parent_rel->pathlist); } else { /* Reject and recycle the new path */ if (!IsA(new_path, IndexPath)) pfree(new_path); } }
目前为止，追踪遇到可困难，此路暂时不通。
考虑其他方向。
这个还是很重要的：
/* * set_plain_rel_pathlist * Build access paths for a plain relation (no subquery, no inheritance) */ static void set_plain_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte) { ///fprintf(stderr,"In set_plain_rel_pathlist \n"); /* Consider sequential scan */ add_path(rel, create_seqscan_path(root, rel, NULL)); /* Consider index scans */ create_index_paths(root, rel); /* Consider TID scans */ create_tidscan_paths(root, rel); /* Now find the cheapest of the paths for this rel */ set_cheapest(rel); }
