Take the 2-minute tour ×
Database Administrators Stack Exchange is a question and answer site for database professionals who wish to improve their database skills and learn from others in the community. It's 100% free, no registration required.

This question is related to a prior one of I asked: Order of columns in a compound index in PostgreSQL (and query order)

Rather than overload that question, I figure I can sharpen and limit my question here. Given the following query (and EXPLAIN ANALYZE), is the compound index I am creating helping?

This first query was run with only simple indexes (a GIST on outline) and a (BTREE on pid).

The query is:

EXPLAIN ANALYZE SELECT DISTINCT ON (path) oid, pid, product_name, type, path, size 
FROM portal.inventory AS inv 
WHERE ST_Intersects(st_geogfromtext('SRID=4326;POLYGON((21.51947021484375 51.55059814453125, 18.9129638671875 51.55059814453125, 18.9129638671875 48.8287353515625, 21.51947021484375 48.8287353515625, 21.51947021484375 51.55059814453125))'), inv.outline) 
AND (inv.pid in (20010,20046)) 

--

The result was the following (which is faster, but perhaps that is only because the database was warm).

"Unique  (cost=581.76..581.76 rows=1 width=89) (actual time=110.436..110.655 rows=249 loops=1)"
"  ->  Sort  (cost=581.76..581.76 rows=1 width=89) (actual time=110.434..110.477 rows=1377 loops=1)"
"        Sort Key: path"
"        Sort Method: quicksort  Memory: 242kB"
"        ->  Bitmap Heap Scan on inventory inv  (cost=577.48..581.75 rows=1 width=89) (actual time=39.257..105.878 rows=1377 loops=1)"
"              Recheck Cond: ((pid = ANY ('{20010,20046}'::integer[])) AND ('0103000020E6100000010000000500000000000000FC843540000000007AC6494000000000B8E93240000000007AC6494000000000B8E9324000000000146A484000000000FC84354000000000146A484000000000FC843540000000007AC64940'::geography && outline))"
"              Rows Removed by Index Recheck: 3731"
"              Filter: (_st_distance('0103000020E6100000010000000500000000000000FC843540000000007AC6494000000000B8E93240000000007AC6494000000000B8E9324000000000146A484000000000FC84354000000000146A484000000000FC843540000000007AC64940'::geography, outline, 0::double precision, false) < 1e-005::double precision)"
"              Rows Removed by Filter: 533"
"              ->  BitmapAnd  (cost=577.48..577.48 rows=1 width=0) (actual time=38.972..38.972 rows=0 loops=1)"
"                    ->  Bitmap Index Scan on inventory_pid_idx  (cost=0.00..123.82 rows=6204 width=0) (actual time=1.116..1.116 rows=7836 loops=1)"
"                          Index Cond: (pid = ANY ('{20010,20046}'::integer[]))"
"                    ->  Bitmap Index Scan on inventory_outline_idx  (cost=0.00..453.41 rows=8212 width=0) (actual time=37.765..37.765 rows=63112 loops=1)"
"                          Index Cond: ('0103000020E6100000010000000500000000000000FC843540000000007AC6494000000000B8E93240000000007AC6494000000000B8E9324000000000146A484000000000FC84354000000000146A484000000000FC843540000000007AC64940'::geography && outline)"
"Total runtime: 110.731 ms"

Now here is the result with the compound index added: (note that the absolute time was slower)

"Unique  (cost=37.81..37.82 rows=1 width=89) (actual time=2464.353..2464.561 rows=249 loops=1)"
"  ->  Sort  (cost=37.81..37.82 rows=1 width=89) (actual time=2464.349..2464.389 rows=1377 loops=1)"
"        Sort Key: path"
"        Sort Method: quicksort  Memory: 242kB"
"        ->  Bitmap Heap Scan on inventory inv  (cost=33.54..37.80 rows=1 width=89) (actual time=2361.018..2459.653 rows=1377 loops=1)"
"              Recheck Cond: (('0103000020E6100000010000000500000000000000FC843540000000007AC6494000000000B8E93240000000007AC6494000000000B8E9324000000000146A484000000000FC84354000000000146A484000000000FC843540000000007AC64940'::geography && outline) AND (pid = ANY ('{20010,20046}'::integer[])))"
"              Filter: (_st_distance('0103000020E6100000010000000500000000000000FC843540000000007AC6494000000000B8E93240000000007AC6494000000000B8E9324000000000146A484000000000FC84354000000000146A484000000000FC843540000000007AC64940'::geography, outline, 0::double precision, false) < 1e-005::double precision)"
"              Rows Removed by Filter: 533"
"              ->  Bitmap Index Scan on inventory_compound_idx  (cost=0.00..33.53 rows=1 width=0) (actual time=2321.684..2321.684 rows=1910 loops=1)"
"                    Index Cond: (('0103000020E6100000010000000500000000000000FC843540000000007AC6494000000000B8E93240000000007AC6494000000000B8E9324000000000146A484000000000FC84354000000000146A484000000000FC843540000000007AC64940'::geography && outline) AND (pid = ANY ('{20010,20046}'::integer[])))"
"Total runtime: 2558.022 ms"

Finally, here is the table definition:

CREATE TABLE portal.inventory
(
  oid bigint,
  product_name character varying(100),
  type character varying(25),
  pid integer,
  size bigint,
  date timestamp without time zone,
  path character varying(200),
  outline geography(Polygon,4326)
)
WITH (
  OIDS=FALSE
);


CREATE INDEX inventory_compound_idx
  ON portal.inventory
  USING gist
  (outline, pid);


CREATE INDEX inventory_outline_idx
  ON portal.inventory
  USING gist
  (outline);


CREATE INDEX inventory_pid_idx
  ON portal.inventory
  USING btree
  (pid);

UPDATE: Answers to questions listed below:

I can adapt the table, but I am trying to keep the rows thin. Your suggestions are varying, types, etc. are things I would like to change.

Basically, each row represents a little bit of meta data about a geospatial image file. We are managing 50M, and this may well grow to hundreds of millions or more. In the DB each file is referenced by a unique OID (sorry for the duplication of the term). They are grouped by "products" where PID is the product ID. There can be about 1,000 OIDs per product. Each image file has a geospatial bounding box (the outline). That is really all that I need for searching. The rest of the data will not be null (type is a text string, size the the file size, date is the date the file was created, and path is a UNC file path to the file).

Now here is why I orderd the query by outline, then PID. Products will be geospatially grouped. So all OID rows for Krakow Poland will be located in the same region physically. So I assume that if I get the bucket down to a small region, the second index will be pretty small (say about 100 products for a city region). Which the IN( ..) clause will pull out.

The actually values of the PIDS were pulled from the other question I posted here. But that table only for products, and thus its size is about 30K, which means quick searches and no need for compound queries.

I wonder if the POSTGreSQL planner is smart enough to decide if a compound index by (outline,pid) is faster than (pid, outline) if both indexes are there. Well I guess I can test.

share|improve this question
1  
Just run each query a couple of times to exclude effects from cold cache. Pick the best of five or something. Also: which of your columns can be NULL? How selective is pid? How many different values / how many rows per value? is your table design fixed, or can you adapt it? –  Erwin Brandstetter Mar 17 at 18:07
    
I would be interested in the resulting time with the suggested index. Or even the output of explain analyze. Would you mind dropping a line here? You could upload the plan to explain.depesz.com and post the link ... –  Erwin Brandstetter Mar 18 at 15:18
    
I am under a bit of time pressure, but I will try to see if I can get some of this. I will address some of your questions as an update. –  Dr.YSG Mar 18 at 19:27
    
added update to the original question, which might answer some of your questions. –  Dr.YSG Mar 18 at 19:40

2 Answers 2

up vote 5 down vote accepted

In a GiST index, the order of columns has a different significance than in a B-tree index. Per documentation:

A multicolumn GiST index can be used with query conditions that involve any subset of the index's columns. Conditions on additional columns restrict the entries returned by the index, but the condition on the first column is the most important one for determining how much of the index needs to be scanned. A GiST index will be relatively ineffective if its first column has only a few distinct values, even if there are many distinct values in additional columns.

In short: put the most selective columns first.

Your EXPLAIN output shows that the condition on pid is more selective (rows=7836) than the one on outline (rows=63112). If that can be generalized (a single example may be misleading) I suggest this alternative:

CREATE INDEX inventory_compound_idx ON portal.inventory USING gist (pid, outline);

If most of your (important) queries include conditions on both columns, a multicolumn index may serve you well. Else, single columns may be better overall.

Table layout

This is an educated guess since I don't have complete information.

  • Don't use oid as column name. It's easy to confuse with the OID.
  • Don't use the name date for a timestamp column. Or rather: don't use the name date for any column, don't use names of base-types for identifiers at all. Can lead to confusing mistakes and error messages.
  • Create a lookup table for types and only put a small integer type_id into the big table. Pack fixed-length types tightly so not to waste space to padding. Details.

  • I prefer the type text (or varchar without length limit) over varchar(n). Details.

For example:

CREATE TABLE portal.inventory (
   inventory_id bigint PRIMARY KEY
  ,type_id      integer NOT NULL REFERENCES inv_type(type_id)
  ,pid          integer NOT NULL
  ,size         bigint NOT NULL
  ,ts           timestamp NOT NULL
  ,outline      geography(Polygon,4326)
  ,product_name text
  ,path         text
);
share|improve this answer
    
I am exceeding pleased with your comments. I wish I could double the vote. Thank you for the help, and help in reading the explain. –  Dr.YSG Mar 18 at 19:24

Erwin, I have some of the data you asked for. This time I tried a more ambitous query (I expect most work will be for a smaller set of pids and a smaller geospatial region) but some folks are going to strain the system, and this might turn out not to biggest query in the end.

SELECT DISTINCT ON (path)  pid, type, path, size 
FROM portal.inventory AS inv 
WHERE ST_Intersects(st_geogfromtext('SRID=4326;POLYGON((21.2310791015625 51.416015625, 18.643798828125 51.416015625, 18.643798828125 48.69415283203125, 21.2310791015625 48.69415283203125, 21.2310791015625 51.416015625))'), inv.outline) 
AND (inv.pid in (23869,23869,23599,23869,14153,14156,110,19131,19131,19164,91,23501,36,23501,23586,23586,23586,23586,23586,23599,23599,20047,113,120,3,120,23,118,82,120,113,113,120,129,129,210,339,339,341,23345,23506,23559,23553,23546,23546,23765,20010,19939,19939,20043,20046,20046,20046,20046,20047,23345,23345,23507,23507,129,23589,23612,23612,23539,23539,23539,23553,23553,23553,23559,23596,23589,23594,23589,23589,23596,23596,23596,23506,23506,23511,23511,23742,23742,23846,23846,23846,23742,23765,23765,341,19939,20047,23612,62,150,150,150,150,150,150,268,268,268,268,23598,120,23501))

Explain analyze for index with (outline, pid)

[What is odd here is that even though the query specifies that WHERE clause has the outline first and the pid second - and thus should be using the index inventory_compound_idx it is using the inverse index icompound_idx ]

"Unique  (cost=10788.37..10792.36 rows=1 width=76) (actual time=3042.605..3120.677 rows=1682 loops=1)"
"  ->  Sort  (cost=10788.37..10790.37 rows=799 width=76) (actual time=3042.600..3114.823 rows=48341 loops=1)"
"        Sort Key: path"
"        Sort Method: external merge  Disk: 4384kB"
"        ->  Bitmap Heap Scan on inventory inv  (cost=503.88..10749.85 rows=799 width=76) (actual time=119.501..2586.973 rows=48341 loops=1)"
"              Recheck Cond: ((pid = ANY ('{23869,23869,23599,23869,14153,14156,110,19131,19131,19164,91,23501,36,23501,23586,23586,23586,23586,23586,23599,23599,20047,113,120,3,120,23,118,82,120,113,113,120,129,129,210,339,339,341,23345,23506,23559,23553,23546,23546,23765,20010,19939,19939,20043,20046,20046,20046,20046,20047,23345,23345,23507,23507,129,23589,23612,23612,23539,23539,23539,23553,23553,23553,23559,23596,23589,23594,23589,23589,23596,23596,23596,23506,23506,23511,23511,23742,23742,23846,23846,23846,23742,23765,23765,341,19939,20047,23612,62,150,150,150,150,150,150,268,268,268,268,23598,120,23501}'::integer[])) AND ('0103000020E6100000010000000500000000000000283B35400000000040B5494000000000D0A432400000000040B5494000000000D0A4324000000000DA58484000000000283B354000000000DA58484000000000283B35400000000040B54940'::geography && outline))"
"              Rows Removed by Index Recheck: 370361"
"              Filter: (_st_distance('0103000020E6100000010000000500000000000000283B35400000000040B5494000000000D0A432400000000040B5494000000000D0A4324000000000DA58484000000000283B354000000000DA58484000000000283B35400000000040B54940'::geography, outline, 0::double precision, false) < 1e-005::double precision)"
"              Rows Removed by Filter: 15439"
"              ->  Bitmap Index Scan on inventory_icompound_idx  (cost=0.00..503.68 rows=2398 width=0) (actual time=117.783..117.783 rows=219595 loops=1)"
"                    Index Cond: ((pid = ANY ('{23869,23869,23599,23869,14153,14156,110,19131,19131,19164,91,23501,36,23501,23586,23586,23586,23586,23586,23599,23599,20047,113,120,3,120,23,118,82,120,113,113,120,129,129,210,339,339,341,23345,23506,23559,23553,23546,23546,23765,20010,19939,19939,20043,20046,20046,20046,20046,20047,23345,23345,23507,23507,129,23589,23612,23612,23539,23539,23539,23553,23553,23553,23559,23596,23589,23594,23589,23589,23596,23596,23596,23506,23506,23511,23511,23742,23742,23846,23846,23846,23742,23765,23765,341,19939,20047,23612,62,150,150,150,150,150,150,268,268,268,268,23598,120,23501}'::integer[])) AND ('0103000020E6100000010000000500000000000000283B35400000000040B5494000000000D0A432400000000040B5494000000000D0A4324000000000DA58484000000000283B354000000000DA58484000000000283B35400000000040B54940'::geography && outline))"
"Total runtime: 3125.598 ms"

And now I switched the query order so that the PID is first in the where clause, so it should use the icompound_idx index (pid, outline).

(Do you see any real advantage, I do not).

"Unique  (cost=10788.37..10792.36 rows=1 width=76) (actual time=3030.431..3108.313 rows=1682 loops=1)"
"  ->  Sort  (cost=10788.37..10790.37 rows=799 width=76) (actual time=3030.429..3102.474 rows=48341 loops=1)"
"        Sort Key: path"
"        Sort Method: external merge  Disk: 4384kB"
"        ->  Bitmap Heap Scan on inventory inv  (cost=503.88..10749.85 rows=799 width=76) (actual time=110.656..2575.282 rows=48341 loops=1)"
"              Recheck Cond: ((pid = ANY ('{23869,23869,23599,23869,14153,14156,110,19131,19131,19164,91,23501,36,23501,23586,23586,23586,23586,23586,23599,23599,20047,113,120,3,120,23,118,82,120,113,113,120,129,129,210,339,339,341,23345,23506,23559,23553,23546,23546,23765,20010,19939,19939,20043,20046,20046,20046,20046,20047,23345,23345,23507,23507,129,23589,23612,23612,23539,23539,23539,23553,23553,23553,23559,23596,23589,23594,23589,23589,23596,23596,23596,23506,23506,23511,23511,23742,23742,23846,23846,23846,23742,23765,23765,341,19939,20047,23612,62,150,150,150,150,150,150,268,268,268,268,23598,120,23501}'::integer[])) AND ('0103000020E6100000010000000500000000000000283B35400000000040B5494000000000D0A432400000000040B5494000000000D0A4324000000000DA58484000000000283B354000000000DA58484000000000283B35400000000040B54940'::geography && outline))"
"              Rows Removed by Index Recheck: 370361"
"              Filter: (_st_distance('0103000020E6100000010000000500000000000000283B35400000000040B5494000000000D0A432400000000040B5494000000000D0A4324000000000DA58484000000000283B354000000000DA58484000000000283B35400000000040B54940'::geography, outline, 0::double precision, false) < 1e-005::double precision)"
"              Rows Removed by Filter: 15439"
"              ->  Bitmap Index Scan on inventory_icompound_idx  (cost=0.00..503.68 rows=2398 width=0) (actual time=109.132..109.132 rows=219595 loops=1)"
"                    Index Cond: ((pid = ANY ('{23869,23869,23599,23869,14153,14156,110,19131,19131,19164,91,23501,36,23501,23586,23586,23586,23586,23586,23599,23599,20047,113,120,3,120,23,118,82,120,113,113,120,129,129,210,339,339,341,23345,23506,23559,23553,23546,23546,23765,20010,19939,19939,20043,20046,20046,20046,20046,20047,23345,23345,23507,23507,129,23589,23612,23612,23539,23539,23539,23553,23553,23553,23559,23596,23589,23594,23589,23589,23596,23596,23596,23506,23506,23511,23511,23742,23742,23846,23846,23846,23742,23765,23765,341,19939,20047,23612,62,150,150,150,150,150,150,268,268,268,268,23598,120,23501}'::integer[])) AND ('0103000020E6100000010000000500000000000000283B35400000000040B5494000000000D0A432400000000040B5494000000000D0A4324000000000DA58484000000000283B354000000000DA58484000000000283B35400000000040B54940'::geography && outline))"
"Total runtime: 3113.334 ms"
share|improve this answer
    
Unlike the order of columns in a index, the order of WHERE clauses is completely irrelevant. SQL is not a procedural language, Postgres applies WHERE clauses as it sees fit. Also note that I found this by chance. I am not notified of additional answers here. –  Erwin Brandstetter Mar 20 at 23:55

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.