#StackBounty: #postgresql #performance #index #postgresql-performance #postgresql-9.5 PostgreSQL – How does multicolumn B-Tree index wo…

Bounty: 100

I created such table (similar to example from http://use-the-index-luke.com/sql/example-schema/postgresql/performance-testing-scalability )

CREATE TABLE scale_data (
   section NUMERIC NOT NULL,
   id1     NUMERIC NOT NULL, -- unique values simulating ID or Timestamp
   id2     NUMERIC NOT NULL -- a kind of Type
);

Populate it with:

INSERT INTO scale_data
SELECT sections.sections, sections.sections*10000 + gen.gen
     , CEIL(RANDOM()*100) 
  FROM GENERATE_SERIES(1, 300)     sections,
       GENERATE_SERIES(1, 90000) gen
 WHERE gen <= sections * 300;

It generated 13545000 records.

Composite index on it:

CREATE INDEX id1_id2_idx
  ON public.scale_data
  USING btree
  (id1, id2);

And select#1:

select id2 from scale_data 
where id2 in (50)
order by id1 desc
limit 500

Explain analyze:

"Limit  (cost=0.56..1177.67 rows=500 width=11) (actual time=0.046..5.124 rows=500 loops=1)"
"  ->  Index Only Scan Backward using id1_id2_idx on scale_data  (cost=0.56..311588.74 rows=132353 width=11) (actual time=0.045..5.060 rows=500 loops=1)"
"        Index Cond: (id2 = '50'::numeric)"
"        Heap Fetches: 0"
"Planning time: 0.103 ms"
"Execution time: 5.177 ms"

Select#2 –more values in IN – plan has changed

select id2 from scale_data 
where id2 in (50, 52)
order by id1 desc
limit 500

Explain analyze#2:

"Limit  (cost=0.56..857.20 rows=500 width=11) (actual time=0.061..8.703 rows=500 loops=1)"
"  ->  Index Only Scan Backward using id1_id2_idx on scale_data  (cost=0.56..445780.74 rows=260190 width=11) (actual time=0.059..8.648 rows=500 loops=1)"
"        Filter: (id2 = ANY ('{50,52}'::numeric[]))"
"        Rows Removed by Filter: 25030"
"        Heap Fetches: 0"
"Planning time: 0.153 ms"
"Execution time: 8.771 ms"

Why plan differs?
Why in #1 it does show like Index condition, but in #2 Filter and number of index scanned cells.
Doesn’t sql#1 traverse index in the same way like explain for sql#2 shows?

On real/production DB #2 works much slower, even if search by 2 keys separately is fast

PG 9.5


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