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README |
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 the RelOptInfo.path_order list. 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 it is joined to 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 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.path_order 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} 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 which relations can do merge sort and hash joins ----find_paths() find scan and all index paths for each relation not yet joined one relation, return find selectivity of columns used in joins -----find_join_paths() jump to geqo if needed again: find_join_rels(): for each joinrel: find_clause_joins() for each join on joinrel: if a join from the join clause adds only one relation, do the join or find_clauseless_joins() find_all_join_paths() generate paths(nested,sortmerge) for joins found in find_join_rels() prune_joinrels() remove from the join list the relation we just added to each join prune_rel_paths() set cheapest and perhaps remove unordered path, recompute table sizes if we have not done all the tables, go to again: do group(GROUP) do aggregate put back constants re-flatten target list make unique(DISTINCT) make sort(ORDER BY) Optimizer Structures -------------------- RelOptInfo - Every relation RestrictInfo - restriction clauses JoinInfo - join combinations Path - every way to access a relation(sequential, index) IndexPath - index scans JoinPath - joins MergePath - merge joins HashPath - hash joins PathOrder - every ordering type (sort, merge of relations)