1

I have a postgres table with the following structure:

+---------+-------------+-------------+----------+---------+---------+
|   id    |  timestamp  |   numvalues |  text1   |  text2  |  text3  |
+---------+-------------+-------------+----------+---------+---------+
|abcd12344|    4124135  |[1,2,53,1241]| apple    | banana  | papaya  |
+---------+-------------+-------------+----------+---------+---------+

id - random alphanumeric value
timestamp - epoch timestamp
numvalues - array of integers
text(n) - text values

The table has about 150 million rows.

I make an inner query to get the nth percentile of the data ordered on one of the values in the array. Then, I need to get the averages of several values from the array. The length of the array is around 31.

SELECT AVG(COALESCE(numvalues[2], 0))::NUMERIC(10,0), AVG(COALESCE(numvalues[3], 0))::NUMERIC(10,0)...AVG(COALESCE(numvalues[12], 0))::NUMERIC(10,0) 
FROM (SELECT timestamp, numvalues, ntile(100) 
      OVER (ORDER BY numvalues[1]) pval FROM tablename WHERE timestamp >= somevalue and timestamp <= somevalue) innertable 
WHERE pval >= x and pval <= y;

This returns about 7 million rows out of 150 million which is about 5% of the table. However, it does not use the index. Running an EXPLAIN ANALYZE shows that it uses a Seq Scan instead, even when enable_seqscan is set to off.

However, a similar query :

SELECT text1, count(distinct(id))
FROM (SELECT timestamp, id, text1, numvalues, ntile(100) 
      OVER (ORDER BY numvalues[1]) pval FROM tablename WHERE timestamp >= somevalue and timestamp <= somevalue) innertable 
WHERE pval >= x and pval <= y GROUP BY text1;

does use the index.

The index is on the timestamp column

The results of EXPLAIN ANALYZE:

explain analyze select text1, count(distinct(id)) 
    from (select timestamp, text1, numvalues, id, ntile(100) over (order by numvalues[1]) pval from table where timestamp >= 1431100800 and timestamp <= 1431108000 and numvalues[1] NOTNULL) innertable 
    where pval >= 90 and pval <= 90 group by text1;

                                  QUERY PLAN

--------------------------------------------------------------------------------
GroupAggregate  (cost=4554118.06..4554641.66 rows=1 width=28) (actual time=218641.221..219051.984 rows=20 loops=1)
   ->  Sort  (cost=4554118.06..4554292.59 rows=69812 width=28) (actual time=218640.546..218728.294 rows=71441 loops=1)
         Sort Key: innertable.text1
         Sort Method: quicksort  Memory: 8654kB
         ->  Subquery Scan on innertable  (cost=4094722.75..4548501.27 rows=69812 width=28) (actual time=216502.946..218521.666 rows=71441 loops=1)
           Filter: ((innertable.pval >= 90) AND (innertable.pval <= 90))
           Rows Removed by Filter: 7072674
           ->  WindowAgg  (cost=4094722.75..4339065.03 rows=13962416 width=118) (actual time=202276.333..211374.235 rows=7144115 loops=1)
                 ->  Sort  (cost=4094722.75..4129628.79 rows=13962416 width=118) (actual time=164912.487..190272.316 rows=7144115 loops=1)
                       Sort Key: (table.numvalues[9])
                       Sort Method: external merge  Disk: 1387704kB
                       ->  Index Scan using table_time_idx on table  (cost=0.57..1578710.87 rows=13962416 width=118) (actual time=0.124..141014.505 rows=7144115 loops=1)
                             Index Cond: (("timestamp" >= 1431100800) AND ("timestamp" <= 1431108000))
                             Filter: (numvalues[1] IS NOT NULL)
                             Rows Removed by Filter: 7090075
 Total runtime: 219340.709 ms
(16 rows)



explain analyze select avg(coalesce(numvalues[9], 0))::NUMERIC(10,0) mean9, avg(coalesce(numvalues[30],0))::NUMERIC(10,0),  avg(coalesce(numvalues[8],0))::NUMERIC(10,0) - avg(coalesce(numvalues[30], 0))::NUMERIC(10,0) mean0, avg(coalesce(numvalues[10],0))::NUMERIC(10,0) - avg(coalesce(numvalues[8], 0))::NUMERIC(10,0) mean1, avg(coalesce(numvalues[14],0))::NUMERIC(10,0) - avg(coalesce(numvalues[10], 0))::NUMERIC(10,0) mean2, avg(coalesce(numvalues[13],0))::NUMERIC(10,0) - avg(coalesce(numvalues[14], 0))::NUMERIC(10,0) mean3, avg(coalesce(numvalues[9],0))::NUMERIC(10,0) - avg(coalesce(numvalues[13], 0))::NUMERIC(10,0) mean4 
    from (select timestamp, id, numvalues, ntile(100) over (order by numvalues[1] ) pval from table where timestamp >= 1431093600.00 and timestamp <= 1431100800.00 and numvalues[9] NOTNULL) innerTable 
    where pval >= 90.00 and pval <= 90.00 ;

QUERY PLAN                                                                        
--------------------------------------------------------------------------------
 Aggregate  (cost=12662077.32..12662077.37 rows=1 width=82) (actual time=650343.769..650343.770 rows=1 loops=1)
   ->  Subquery Scan on innertable  (cost=12634854.54..12661968.84 rows=3615 width=82) (actual time=647745.962..650232.725 rows=71441 loops=1)
         Filter: (((innertable.pval)::numeric >= 90.00) AND ((innertable.pval)::numeric <= 90.00))
         Rows Removed by Filter: 7072674
          ->  WindowAgg  (cost=12634854.54..12647507.88 rows=723048 width=248) (actual time=632388.293..642338.237 rows=7144115 loops=1)
           ->  Sort  (cost=12634854.54..12636662.16 rows=723048 width=248) (actual time=599893.771..617413.102 rows=7144115 loops=1)
                 Sort Key: (table.numvalues[9])
                 Sort Method: external merge  Disk: 3214248kB
                 ->  Seq Scan on req_p0swajch2t  (cost=0.00..12480460.48 rows=723048 width=248) (actual time=0.041..575423.062 rows=7144115 loops=1)
                       Filter: ((numvalues[1] IS NOT NULL) AND (("timestamp")::numeric >= 1431100800.00) AND (("timestamp")::numeric <= 1431108000.00))
                       Rows Removed by Filter: 138191935
Total runtime: 650785.126 ms
(12 rows)

Can anyone help me out with why Postgres would use an index for one query and not the other? Running a VACUUM ANALYZE didn't help either.

Is there any way to speed up the queries? A query over the whole table would take about 20 - 30 minutes! Partitioning didn't make much difference since queries span multiple partitions and it showed an improvement of only about a couple of minutes when the number of partitions spanned became larger.

1

From your query plans, it looks like you're comparing ints to ints in the first query plan, and int to numeric in the second plan.

Your first compare:

Index Cond: (("timestamp" >= 1431100800) AND ("timestamp" <= 1431108000))

and

timestamp >= 1431100800 and timestamp <= 1431108000

In the second query, it's numeric values:

Filter: ((numvalues[1] IS NOT NULL) AND (("timestamp")::numeric >= 1431100800.00) AND (("timestamp")::numeric <= 1431108000.00))

and

timestamp >= 1431093600.00 and timestamp <= 1431100800.00

Casting to numeric causes the index to be ignored in favor of a sequential scan.

You can see this with a very simple example, set up below:

CREATE TABLE t2 (a int);
CREATE INDEX t2_a_idx ON t2(a);
INSERT INTO t2 (a) SELECT i FROM generate_series(1,1000000) AS i;
VACUUM ANALYZE VERBOSE t2;

My first query plan looks like this:

EXPLAIN ANALYZE SELECT * FROM t2 WHERE a > 750000;

Index Only Scan using t2_a_idx on t2 (cost=0.42..7134.65 rows=250413 width=4) 
(actual time=0.019..29.926 rows=250000 loops=1)
Index Cond: (a > 750000)
Heap Fetches: 0
Planning time: 0.137 ms
Execution time: 39.114 ms
(5 rows)
Time: 39.540 ms

While a second query using numerics looks like this:

EXPLAIN ANALYZE SELECT * FROM t2 WHERE a > 750000.00;

Seq Scan on t2  (cost=0.00..19425.00 rows=333333 width=4) (actual time=122.803..175.326 rows=250000 loops=1)
Filter: ((a)::numeric > 750000.00)
Rows Removed by Filter: 750000
Planning time: 0.058 ms
Execution time: 184.194 ms
(5 rows)
Time: 184.487 ms

In the second instance here, the index is ignored in favor of a sequential scan because of the cast to a numeric value, which looks like exactly what's happening in your two examples.

One last aside, you might be able to speed your query up via a SET query before executing it as well:

SET work_mem = 2GB;

If your server can handle it, because your sorts are spilling to disk, as noted in this line from your query plan:

Sort Method: external merge  Disk: 1387704kB

Hope this helps. =)

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