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67 lines
2.3 KiB
Plaintext
67 lines
2.3 KiB
Plaintext
Thse directories take the Query structure returned by the parser, and
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generate a plan used by the executor. The /plan directory generates the
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plan, the /path generates all possible ways to join the tables, and
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/prep handles special cases like inheritance. /utils is utility stuff.
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planner()
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handle inheritance by processing separately
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-init_query_planner()
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preprocess target list
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preprocess qualifications(WHERE)
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--query_planner()
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pull out constants from target list
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get a target list that only contains column names, no expressions
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if none, then return
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---subplanner()
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make list of relations in target
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make list of relations in where clause
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find which relations can do merge sort and hash joins
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----find_paths()
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find scan and all index paths for each relation not yet joined
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one relation, return
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find selectivity of columns used in joins
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-----find_join_paths()
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Summary: With OPTIMIZER_DEBUG defined, you see:
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Tables 1, 2, 3, and 4 are joined as:
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{1 2},{1 3},{1 4},{2 3},{2 4}
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{1 2 3},{1 2 4},{2 3 4}
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{1 2 3 4}
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Actual output tests show combinations:
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{4 2},{3 2},{1 4},{1 3},{1 2}
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{4 2 3},{1 4 2},{1 3 2}
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{4 2 3 1}
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Cheapest join order shows:
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{4 2},{3 2},{1 4},{1 3},{1 2}
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{3 2 4},{1 4 2},{1 3 2}
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{1 4 2 3}
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It first finds the best way to join each table to every other
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table. It then takes those joined table combinations, and joins
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them to the other joined table combinations, until all tables are
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joined.
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jump to geqo if needed
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again:
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find_join_rels():
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for each joinrel:
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find_clause_joins()
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for each join on joinrel:
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if a join from the join clause adds only one relation, do the join
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or find_clauseless_joins()
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find_all_join_paths()
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generate paths(nested,mergesort) for joins found in find_join_rels()
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prune_joinrels()
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remove from the join list the relation we just added to each join
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prune_rel_paths()
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set cheapest and perhaps remove unordered path, recompute table sizes
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if we have not done all the tables, go to "again"
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do group(GROUP)
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do aggregate
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put back constants
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re-flatten target list
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make unique(DISTINCT)
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make sort(ORDER BY)
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