Summary ------- The optimizer generates optimial query plans by doing several steps: 1) Take each relation in a query, and make a RelOptInfo structure for it. Find each way of accessing the relation, called a Path, including sequential and index scans, and add it to RelOptInfo.pathlist. 2) Join each RelOptInfo to each other RelOptInfo as specified in the WHERE clause. At this point each RelOptInfo is a single relation, so you are joining every relation to every relation as joined in the WHERE clause. Joins occur using two RelOptInfos. One is outer, the other inner. Outers drive lookups of values in the inner. In a nested loop, lookups of values in the inner occur by scanning to find each matching inner row. In a mergejoin, inner and outer rows are ordered, and are accessed in order, so only one scan of inner is required to perform the entire join. In a hashjoin, inner rows are hashed for lookups. Each unique join combination becomes a new RelOptInfo. The RelOptInfo is now the joining of two relations. RelOptInfo.pathlist are various paths to create the joined result, having different orderings depending on the join method used. 3) At this point, every RelOptInfo is joined to each other again, with a new relation added to each RelOptInfo. This continues until all relations have been joined into one RelOptInfo, and the cheapest Path is chosen. SELECT * FROM tab1, tab2, tab3, tab4 WHERE tab1.col = tab2.col AND tab2.col = tab3.col AND tab3.col = tab4.col Tables 1, 2, 3, and 4 are joined as: {1 2},{2 3},{3 4} {1 2 3},{2 3 4} {1 2 3 4} SELECT * FROM tab1, tab2, tab3, tab4 WHERE tab1.col = tab2.col AND tab1.col = tab3.col AND tab1.col = tab4.col Tables 1, 2, 3, and 4 are joined as: {1 2},{1 3},{1 4} {1 2 3},{1 3 4},{1,2,4} {1 2 3 4} In the default left-handed joins, each RelOptInfo adds one single-relation RelOptInfo in each join pass, and the added RelOptInfo is always the inner relation in the join. In right-handed joins, the added RelOptInfo is the outer relation in the join. In bushy plans, multi-relation RelOptInfo's can be joined to other multi-relation RelOptInfo's. Optimizer Functions ------------------- These directories take the Query structure returned by the parser, and generate a plan used by the executor. The /plan directory generates the plan, the /path generates all possible ways to join the tables, and /prep handles special cases like inheritance. /utils is utility stuff. planner() handle inheritance by processing separately -init_query_planner() preprocess target list preprocess qualifications(WHERE) --query_planner() cnfify() Summary: Simple cases with all AND's are handled by removing the AND's: convert: a = 1 AND b = 2 AND c = 3 to: a = 1, b = 2, c = 3 Qualifications with OR's are handled differently. OR's inside AND clauses are not modified drastically: convert: a = 1 AND b = 2 AND (c = 3 OR d = 4) to: a = 1, b = 2, c = 3 OR d = 4 OR's in the upper level are more complex to handle: convert: (a = 1 AND b = 2) OR c = 3 to: (a = 1 OR c = 3) AND (b = 2 OR c = 3) finally: (a = 1 OR c = 3), (b = 2 OR c = 3) These clauses all have to be true for a result to be returned, so the optimizer can choose the most restrictive clauses. pull out constants from target list get a target list that only contains column names, no expressions if none, then return ---subplanner() make list of relations in target make list of relations in where clause split up the qual into restrictions (a=1) and joins (b=c) find relation clauses can do merge sort and hash joins ----make_one_rel() set_base_rel_pathlist() find scan and all index paths for each relation find selectivity of columns used in joins -----make_one_rel_by_joins() jump to geqo if needed again: make_rels_by_joins(): for each joinrel: make_rels_by_clause_joins() for each rel's joininfo list: if a join from the join clause adds only one relation, do the join or make_rels_by_clauseless_joins() update_rels_pathlist_for_joins() generate nested,merge,hash join paths for new rel's created above merge_rels_with_same_relids() merge RelOptInfo paths that have the same relids because of joins rels_set_cheapest() set cheapest path if all relations in one RelOptInfo, return do group(GROUP) do aggregate put back constants re-flatten target list make unique(DISTINCT) make sort(ORDER BY) Optimizer Structures -------------------- RelOptInfo - a relation or joined relations RestrictInfo - restriction clauses JoinInfo - join clauses Path - every way to generate a RelOptInfo(sequential,index,joins) IndexPath - index scans NestPath - nested joins MergePath - merge joins HashPath - hash joins PathOrder - every ordering type (sort, merge of relations)
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Bruce Momjian authoredcd550c76