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author | Bruce Momjian <bruce@momjian.us> | 1997-12-17 18:02:33 +0000 |
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committer | Bruce Momjian <bruce@momjian.us> | 1997-12-17 18:02:33 +0000 |
commit | d158fce8eb942c9d42556479e68110c24c1d15da (patch) | |
tree | 7d5acf65f5fb20262c3bb596b0762a8eee7aba1d /src/backend/optimizer/README | |
parent | 542d4e528d9622b2f0d0f9444fb963a9c6b1c209 (diff) | |
download | postgresql-d158fce8eb942c9d42556479e68110c24c1d15da.tar.gz |
Add optimizer README file.
Diffstat (limited to 'src/backend/optimizer/README')
-rw-r--r-- | src/backend/optimizer/README | 66 |
1 files changed, 66 insertions, 0 deletions
diff --git a/src/backend/optimizer/README b/src/backend/optimizer/README new file mode 100644 index 0000000000..d9ced5d539 --- /dev/null +++ b/src/backend/optimizer/README @@ -0,0 +1,66 @@ +Thse 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() + 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 + 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() + Summary: With OPTIMIZER_DEBUG defined, you see: + + Tables 1, 2, 3, and 4 are joined as: + {1 2},{1 3},{1 4},{2 3},{2 4} + {1 2 3},{1 2 4},{2 3 4} + {1 2 3 4} + + Actual output tests show combinations: + {4 2},{3 2},{1 4},{1 3},{1 2} + {4 2 3},{1 4 2},{1 3 2} + {4 2 3 1} + + Cheapest join order shows: + {4 2},{3 2},{1 4},{1 3},{1 2} + {3 2 4},{1 4 2},{1 3 2} + {1 4 2 3} + + It first finds the best way to join each table to every other + table. It then takes those joined table combinations, and joins + them to the other joined table combinations, until all tables are + joined. + + 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,mergesort) 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) |