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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
68 lines
2.6 KiB
SQL
68 lines
2.6 KiB
SQL
LOAD 'pg_plan_advice';
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-- An empty string is allowed. Empty target lists are allowed for most advice
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-- tags, but not for JOIN_ORDER. "Supplied Plan Advice" should be omitted in
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-- text format when there is no actual advice, but not in non-text format.
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SET pg_plan_advice.advice = '';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = 'SEQ_SCAN()';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = 'NESTED_LOOP_PLAIN()';
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EXPLAIN (COSTS OFF, FORMAT JSON) SELECT 1;
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SET pg_plan_advice.advice = 'JOIN_ORDER()';
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-- Test assorted variations in capitalization, whitespace, and which parts of
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-- the relation identifier are included. These should all work.
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SET pg_plan_advice.advice = 'SEQ_SCAN(x)';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = 'seq_scan(x@y)';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = 'SEQ_scan(x#2)';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = 'SEQ_SCAN (x/y)';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = ' SEQ_SCAN ( x / y . z ) ';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = 'SEQ_SCAN("x"#2/"y"."z"@"t")';
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EXPLAIN (COSTS OFF) SELECT 1;
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-- Syntax errors.
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SET pg_plan_advice.advice = 'SEQUENTIAL_SCAN(x)';
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SET pg_plan_advice.advice = 'SEQ_SCAN';
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SET pg_plan_advice.advice = 'SEQ_SCAN(';
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SET pg_plan_advice.advice = 'SEQ_SCAN("';
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SET pg_plan_advice.advice = 'SEQ_SCAN("")';
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SET pg_plan_advice.advice = 'SEQ_SCAN("a"';
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SET pg_plan_advice.advice = 'SEQ_SCAN(#';
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SET pg_plan_advice.advice = '()';
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SET pg_plan_advice.advice = '123';
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-- Tags like SEQ_SCAN and NO_GATHER don't allow sublists at all; other tags,
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-- except for JOIN_ORDER, allow at most one level of sublist. Hence, these
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-- examples should error out.
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SET pg_plan_advice.advice = 'SEQ_SCAN((x))';
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SET pg_plan_advice.advice = 'GATHER(((x)))';
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-- Legal comments.
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SET pg_plan_advice.advice = '/**/';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = 'HASH_JOIN(_)/***/';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = '/* comment */ HASH_JOIN(/*x*/y)';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = '/* comment */ HASH_JOIN(y//*x*/z)';
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EXPLAIN (COSTS OFF) SELECT 1;
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-- Unterminated comments.
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SET pg_plan_advice.advice = '/*';
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SET pg_plan_advice.advice = 'JOIN_ORDER("fOO") /* oops';
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-- Nested comments are not supported, so the first of these is legal and
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-- the second is not.
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SET pg_plan_advice.advice = '/*/*/';
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EXPLAIN (COSTS OFF) SELECT 1;
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SET pg_plan_advice.advice = '/*/* stuff */*/';
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-- Foreign join requires multiple relation identifiers.
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SET pg_plan_advice.advice = 'FOREIGN_JOIN(a)';
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SET pg_plan_advice.advice = 'FOREIGN_JOIN((a))';
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