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Provide a facility that (1) can be used to stabilize certain plan choices so that the planner cannot reverse course without authorization and (2) can be used by knowledgeable users to insist on plan choices contrary to what the planner believes best. In both cases, terrible outcomes are possible: users should think twice and perhaps three times before constraining the planner's ability to do as it thinks best; nevertheless, there are problems that are much more easily solved with these facilities than without them. This patch takes the approach of analyzing a finished plan to produce textual output, which we call "plan advice", that describes key decisions made during plan; if that plan advice is provided during future planning cycles, it will force those key decisions to be made in the same way. Not all planner decisions can be controlled using advice; for example, decisions about how to perform aggregation are currently out of scope, as is choice of sort order. Plan advice can also be edited by the user, or even written from scratch in simple cases, making it possible to generate outcomes that the planner would not have produced. Partial advice can be provided to control some planner outcomes but not others. Currently, plan advice is focused only on specific outcomes, such as the choice to use a sequential scan for a particular relation, and not on estimates that might contribute to those outcomes, such as a possibly-incorrect selectivity estimate. While it would be useful to users to be able to provide plan advice that affects selectivity estimates or other aspects of costing, that is out of scope for this commit. Reviewed-by: Lukas Fittl <lukas@fittl.com> Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.com> Reviewed-by: Greg Burd <greg@burd.me> Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com> Reviewed-by: Haibo Yan <tristan.yim@gmail.com> Reviewed-by: Dian Fay <di@nmfay.com> Reviewed-by: Ajay Pal <ajay.pal.k@gmail.com> Reviewed-by: John Naylor <johncnaylorls@gmail.com> Reviewed-by: Alexandra Wang <alexandra.wang.oss@gmail.com> Discussion: http://postgr.es/m/CA+TgmoZ-Jh1T6QyWoCODMVQdhTUPYkaZjWztzP1En4=ZHoKPzw@mail.gmail.com
86 lines
3.3 KiB
PL/PgSQL
86 lines
3.3 KiB
PL/PgSQL
LOAD 'pg_plan_advice';
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SET max_parallel_workers_per_gather = 1;
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SET parallel_setup_cost = 0;
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SET parallel_tuple_cost = 0;
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SET min_parallel_table_scan_size = 0;
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SET debug_parallel_query = off;
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CREATE TABLE gt_dim (id serial primary key, dim text)
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WITH (autovacuum_enabled = false);
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INSERT INTO gt_dim (dim) SELECT random()::text FROM generate_series(1,100) g;
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VACUUM ANALYZE gt_dim;
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CREATE TABLE gt_fact (
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id int not null,
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dim_id integer not null references gt_dim (id)
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) WITH (autovacuum_enabled = false);
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INSERT INTO gt_fact
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SELECT g, (g%3)+1 FROM generate_series(1,100000) g;
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VACUUM ANALYZE gt_fact;
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-- By default, we expect Gather Merge with a parallel hash join.
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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-- Force Gather or Gather Merge of both relations together.
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BEGIN;
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SET LOCAL pg_plan_advice.advice = 'gather_merge((f d))';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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SET LOCAL pg_plan_advice.advice = 'gather((f d))';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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COMMIT;
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-- Force a separate Gather or Gather Merge operation for each relation.
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BEGIN;
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SET LOCAL pg_plan_advice.advice = 'gather_merge(f d)';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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SET LOCAL pg_plan_advice.advice = 'gather(f d)';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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SET LOCAL pg_plan_advice.advice = 'gather((d d/d.d))';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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COMMIT;
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-- Force a Gather or Gather Merge on one relation but no parallelism on other.
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BEGIN;
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SET LOCAL pg_plan_advice.advice = 'gather_merge(f) no_gather(d)';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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SET LOCAL pg_plan_advice.advice = 'gather_merge(d) no_gather(f)';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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SET LOCAL pg_plan_advice.advice = 'gather(f) no_gather(d)';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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SET LOCAL pg_plan_advice.advice = 'gather(d) no_gather(f)';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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COMMIT;
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-- Force no Gather or Gather Merge use at all.
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BEGIN;
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SET LOCAL pg_plan_advice.advice = 'no_gather(f d)';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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COMMIT;
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-- Can't force Gather Merge without the ORDER BY clause, but just Gather is OK.
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BEGIN;
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SET LOCAL pg_plan_advice.advice = 'gather_merge((f d))';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id;
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SET LOCAL pg_plan_advice.advice = 'gather((f d))';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id;
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COMMIT;
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-- Test conflicting advice.
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BEGIN;
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SET LOCAL pg_plan_advice.advice = 'gather((f d)) no_gather(f)';
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EXPLAIN (COSTS OFF, PLAN_ADVICE)
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SELECT * FROM gt_fact f JOIN gt_dim d ON f.dim_id = d.id ORDER BY d.id;
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COMMIT;
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