mirror of
https://github.com/postgres/postgres.git
synced 2026-05-23 10:46:57 -04:00
c2a4078eba
The topic of turning EXPLAIN's BUFFERS option on with the ANALYZE option has come up a few times over the past few years. In many ways, doing this seems like a good idea as it may be more obvious to users why a given query is running more slowly than they might expect. Also, from my own (David's) personal experience, I've seen users posting to the mailing lists with two identical plans, one slow and one fast asking why their query is sometimes slow. In many cases, this is due to additional reads. Having BUFFERS on by default may help reduce some of these questions, and if not, make it more obvious to the user before they post, or save a round-trip to the mailing list when additional I/O effort is the cause of the slowness. The general consensus is that we want BUFFERS on by default with ANALYZE. However, there were more than zero concerns raised with doing so. The primary reason against is the additional verbosity, making it harder to read large plans. Another concern was that buffer information isn't always useful so may not make sense to have it on by default. It's currently December, so let's commit this to see if anyone comes forward with a strong objection against making this change. We have over half a year remaining in the v18 cycle where we could still easily consider reverting this if someone were to come forward with a convincing enough reason as to why doing this is a bad idea. There were two patches independently submitted to achieve this goal, one by me and the other by Guillaume. This commit is a mix of both of these patches with some additional work done by me to adjust various additional places in the documentation which include EXPLAIN ANALYZE output. Author: Guillaume Lelarge, David Rowley Reviewed-by: Robert Haas, Greg Sabino Mullane, Michael Christofides Discussion: https://postgr.es/m/CANNMO++W7MM8T0KyXN3ZheXXt-uLVM3aEtZd+WNfZ=obxffUiA@mail.gmail.com
300 lines
13 KiB
PL/PgSQL
300 lines
13 KiB
PL/PgSQL
-- When there is a LIMIT clause, incremental sort is beneficial because
|
|
-- it only has to sort some of the groups, and not the entire table.
|
|
explain (costs off)
|
|
select * from (select * from tenk1 order by four) t order by four, ten
|
|
limit 1;
|
|
|
|
-- When work_mem is not enough to sort the entire table, incremental sort
|
|
-- may be faster if individual groups still fit into work_mem.
|
|
set work_mem to '2MB';
|
|
explain (costs off)
|
|
select * from (select * from tenk1 order by four) t order by four, ten;
|
|
reset work_mem;
|
|
|
|
create table t(a integer, b integer);
|
|
|
|
create or replace function explain_analyze_without_memory(query text)
|
|
returns table (out_line text) language plpgsql
|
|
as
|
|
$$
|
|
declare
|
|
line text;
|
|
begin
|
|
for line in
|
|
execute 'explain (analyze, costs off, summary off, timing off, buffers off) ' || query
|
|
loop
|
|
out_line := regexp_replace(line, '\d+kB', 'NNkB', 'g');
|
|
return next;
|
|
end loop;
|
|
end;
|
|
$$;
|
|
|
|
create or replace function explain_analyze_inc_sort_nodes(query text)
|
|
returns jsonb language plpgsql
|
|
as
|
|
$$
|
|
declare
|
|
elements jsonb;
|
|
element jsonb;
|
|
matching_nodes jsonb := '[]'::jsonb;
|
|
begin
|
|
execute 'explain (analyze, costs off, summary off, timing off, buffers off, format ''json'') ' || query into strict elements;
|
|
while jsonb_array_length(elements) > 0 loop
|
|
element := elements->0;
|
|
elements := elements - 0;
|
|
case jsonb_typeof(element)
|
|
when 'array' then
|
|
if jsonb_array_length(element) > 0 then
|
|
elements := elements || element;
|
|
end if;
|
|
when 'object' then
|
|
if element ? 'Plan' then
|
|
elements := elements || jsonb_build_array(element->'Plan');
|
|
element := element - 'Plan';
|
|
else
|
|
if element ? 'Plans' then
|
|
elements := elements || jsonb_build_array(element->'Plans');
|
|
element := element - 'Plans';
|
|
end if;
|
|
if (element->>'Node Type')::text = 'Incremental Sort' then
|
|
matching_nodes := matching_nodes || element;
|
|
end if;
|
|
end if;
|
|
end case;
|
|
end loop;
|
|
return matching_nodes;
|
|
end;
|
|
$$;
|
|
|
|
create or replace function explain_analyze_inc_sort_nodes_without_memory(query text)
|
|
returns jsonb language plpgsql
|
|
as
|
|
$$
|
|
declare
|
|
nodes jsonb := '[]'::jsonb;
|
|
node jsonb;
|
|
group_key text;
|
|
space_key text;
|
|
begin
|
|
for node in select * from jsonb_array_elements(explain_analyze_inc_sort_nodes(query)) t loop
|
|
for group_key in select unnest(array['Full-sort Groups', 'Pre-sorted Groups']::text[]) t loop
|
|
for space_key in select unnest(array['Sort Space Memory', 'Sort Space Disk']::text[]) t loop
|
|
node := jsonb_set(node, array[group_key, space_key, 'Average Sort Space Used'], '"NN"', false);
|
|
node := jsonb_set(node, array[group_key, space_key, 'Peak Sort Space Used'], '"NN"', false);
|
|
end loop;
|
|
end loop;
|
|
nodes := nodes || node;
|
|
end loop;
|
|
return nodes;
|
|
end;
|
|
$$;
|
|
|
|
create or replace function explain_analyze_inc_sort_nodes_verify_invariants(query text)
|
|
returns bool language plpgsql
|
|
as
|
|
$$
|
|
declare
|
|
node jsonb;
|
|
group_stats jsonb;
|
|
group_key text;
|
|
space_key text;
|
|
begin
|
|
for node in select * from jsonb_array_elements(explain_analyze_inc_sort_nodes(query)) t loop
|
|
for group_key in select unnest(array['Full-sort Groups', 'Pre-sorted Groups']::text[]) t loop
|
|
group_stats := node->group_key;
|
|
for space_key in select unnest(array['Sort Space Memory', 'Sort Space Disk']::text[]) t loop
|
|
if (group_stats->space_key->'Peak Sort Space Used')::bigint < (group_stats->space_key->'Peak Sort Space Used')::bigint then
|
|
raise exception '% has invalid max space < average space', group_key;
|
|
end if;
|
|
end loop;
|
|
end loop;
|
|
end loop;
|
|
return true;
|
|
end;
|
|
$$;
|
|
|
|
-- A single large group tested around each mode transition point.
|
|
insert into t(a, b) select i/100 + 1, i + 1 from generate_series(0, 999) n(i);
|
|
analyze t;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 31;
|
|
select * from (select * from t order by a) s order by a, b limit 31;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 32;
|
|
select * from (select * from t order by a) s order by a, b limit 32;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 33;
|
|
select * from (select * from t order by a) s order by a, b limit 33;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 65;
|
|
select * from (select * from t order by a) s order by a, b limit 65;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 66;
|
|
select * from (select * from t order by a) s order by a, b limit 66;
|
|
delete from t;
|
|
|
|
-- An initial large group followed by a small group.
|
|
insert into t(a, b) select i/50 + 1, i + 1 from generate_series(0, 999) n(i);
|
|
analyze t;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 55;
|
|
select * from (select * from t order by a) s order by a, b limit 55;
|
|
-- Test EXPLAIN ANALYZE with only a fullsort group.
|
|
select explain_analyze_without_memory('select * from (select * from t order by a) s order by a, b limit 55');
|
|
select jsonb_pretty(explain_analyze_inc_sort_nodes_without_memory('select * from (select * from t order by a) s order by a, b limit 55'));
|
|
select explain_analyze_inc_sort_nodes_verify_invariants('select * from (select * from t order by a) s order by a, b limit 55');
|
|
delete from t;
|
|
|
|
-- An initial small group followed by a large group.
|
|
insert into t(a, b) select (case when i < 5 then i else 9 end), i from generate_series(1, 1000) n(i);
|
|
analyze t;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 70;
|
|
select * from (select * from t order by a) s order by a, b limit 70;
|
|
-- Checks case where we hit a group boundary at the last tuple of a batch.
|
|
-- Because the full sort state is bounded, we scan 64 tuples (the mode
|
|
-- transition point) but only retain 5. Thus when we transition modes, all
|
|
-- tuples in the full sort state have different prefix keys.
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 5;
|
|
select * from (select * from t order by a) s order by a, b limit 5;
|
|
|
|
-- Test rescan.
|
|
begin;
|
|
-- We force the planner to choose a plan with incremental sort on the right side
|
|
-- of a nested loop join node. That way we trigger the rescan code path.
|
|
set local enable_hashjoin = off;
|
|
set local enable_mergejoin = off;
|
|
set local enable_material = off;
|
|
set local enable_sort = off;
|
|
explain (costs off) select * from t left join (select * from (select * from t order by a) v order by a, b) s on s.a = t.a where t.a in (1, 2);
|
|
select * from t left join (select * from (select * from t order by a) v order by a, b) s on s.a = t.a where t.a in (1, 2);
|
|
rollback;
|
|
-- Test EXPLAIN ANALYZE with both fullsort and presorted groups.
|
|
select explain_analyze_without_memory('select * from (select * from t order by a) s order by a, b limit 70');
|
|
select jsonb_pretty(explain_analyze_inc_sort_nodes_without_memory('select * from (select * from t order by a) s order by a, b limit 70'));
|
|
select explain_analyze_inc_sort_nodes_verify_invariants('select * from (select * from t order by a) s order by a, b limit 70');
|
|
delete from t;
|
|
|
|
-- Small groups of 10 tuples each tested around each mode transition point.
|
|
insert into t(a, b) select i / 10, i from generate_series(1, 1000) n(i);
|
|
analyze t;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 31;
|
|
select * from (select * from t order by a) s order by a, b limit 31;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 32;
|
|
select * from (select * from t order by a) s order by a, b limit 32;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 33;
|
|
select * from (select * from t order by a) s order by a, b limit 33;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 65;
|
|
select * from (select * from t order by a) s order by a, b limit 65;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 66;
|
|
select * from (select * from t order by a) s order by a, b limit 66;
|
|
delete from t;
|
|
|
|
-- Small groups of only 1 tuple each tested around each mode transition point.
|
|
insert into t(a, b) select i, i from generate_series(1, 1000) n(i);
|
|
analyze t;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 31;
|
|
select * from (select * from t order by a) s order by a, b limit 31;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 32;
|
|
select * from (select * from t order by a) s order by a, b limit 32;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 33;
|
|
select * from (select * from t order by a) s order by a, b limit 33;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 65;
|
|
select * from (select * from t order by a) s order by a, b limit 65;
|
|
explain (costs off) select * from (select * from t order by a) s order by a, b limit 66;
|
|
select * from (select * from t order by a) s order by a, b limit 66;
|
|
delete from t;
|
|
|
|
drop table t;
|
|
|
|
-- Incremental sort vs. parallel queries
|
|
set min_parallel_table_scan_size = '1kB';
|
|
set min_parallel_index_scan_size = '1kB';
|
|
set parallel_setup_cost = 0;
|
|
set parallel_tuple_cost = 0;
|
|
set max_parallel_workers_per_gather = 2;
|
|
|
|
create table t (a int, b int, c int);
|
|
insert into t select mod(i,10),mod(i,10),i from generate_series(1,10000) s(i);
|
|
create index on t (a);
|
|
analyze t;
|
|
|
|
set enable_incremental_sort = off;
|
|
explain (costs off) select a,b,sum(c) from t group by 1,2 order by 1,2,3 limit 1;
|
|
|
|
set enable_incremental_sort = on;
|
|
explain (costs off) select a,b,sum(c) from t group by 1,2 order by 1,2,3 limit 1;
|
|
|
|
-- Incremental sort vs. set operations with varno 0
|
|
set enable_hashagg to off;
|
|
explain (costs off) select * from t union select * from t order by 1,3;
|
|
|
|
-- Full sort, not just incremental sort can be pushed below a gather merge path
|
|
-- by generate_useful_gather_paths.
|
|
explain (costs off) select distinct a,b from t;
|
|
|
|
drop table t;
|
|
|
|
-- Sort pushdown can't go below where expressions are part of the rel target.
|
|
-- In particular this is interesting for volatile expressions which have to
|
|
-- go above joins since otherwise we'll incorrectly use expression evaluations
|
|
-- across multiple rows.
|
|
set enable_hashagg=off;
|
|
set enable_seqscan=off;
|
|
set enable_incremental_sort = off;
|
|
set parallel_tuple_cost=0;
|
|
set parallel_setup_cost=0;
|
|
set min_parallel_table_scan_size = 0;
|
|
set min_parallel_index_scan_size = 0;
|
|
|
|
-- Parallel sort below join.
|
|
explain (costs off) select distinct sub.unique1, stringu1
|
|
from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub;
|
|
explain (costs off) select sub.unique1, stringu1
|
|
from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub
|
|
order by 1, 2;
|
|
-- Parallel sort but with expression that can be safely generated at the base rel.
|
|
explain (costs off) select distinct sub.unique1, md5(stringu1)
|
|
from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub;
|
|
explain (costs off) select sub.unique1, md5(stringu1)
|
|
from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub
|
|
order by 1, 2;
|
|
-- Parallel sort with an aggregate that can be safely generated in parallel,
|
|
-- but we can't sort by partial aggregate values.
|
|
explain (costs off) select count(*)
|
|
from tenk1 t1
|
|
join tenk1 t2 on t1.unique1 = t2.unique2
|
|
join tenk1 t3 on t2.unique1 = t3.unique1
|
|
order by count(*);
|
|
-- Parallel sort but with expression (correlated subquery) that
|
|
-- is prohibited in parallel plans.
|
|
explain (costs off) select distinct
|
|
unique1,
|
|
(select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
|
|
from tenk1 t, generate_series(1, 1000);
|
|
explain (costs off) select
|
|
unique1,
|
|
(select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
|
|
from tenk1 t, generate_series(1, 1000)
|
|
order by 1, 2;
|
|
-- Parallel sort but with expression not available until the upper rel.
|
|
explain (costs off) select distinct sub.unique1, stringu1 || random()::text
|
|
from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub;
|
|
explain (costs off) select sub.unique1, stringu1 || random()::text
|
|
from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub
|
|
order by 1, 2;
|
|
|
|
reset enable_hashagg;
|
|
reset enable_seqscan;
|
|
reset enable_incremental_sort;
|
|
reset parallel_tuple_cost;
|
|
reset parallel_setup_cost;
|
|
reset min_parallel_table_scan_size;
|
|
reset min_parallel_index_scan_size;
|
|
|
|
-- Ensure incremental sorts work for amcanorderbyop type indexes
|
|
create table point_table (a point, b int);
|
|
create index point_table_a_idx on point_table using gist(a);
|
|
|
|
-- Ensure we get an incremental sort plan for both of the following queries
|
|
explain (costs off) select a, b, a <-> point(5, 5) dist from point_table order by dist, b limit 1;
|
|
explain (costs off) select a, b, a <-> point(5, 5) dist from point_table order by dist, b desc limit 1;
|
|
|
|
-- Ensure we get an incremental sort on the outer side of the mergejoin
|
|
explain (costs off)
|
|
select * from
|
|
(select * from tenk1 order by four) t1 join tenk1 t2 on t1.four = t2.four and t1.two = t2.two
|
|
order by t1.four, t1.two limit 1;
|