Postgresql – Should database be denormalized if I need an index across multiple tables

indexnormalizationpostgresqlpostgresql-9.3

I have a catalog of album and song names. The song has a foreign key to the album table. I expect queries like:

SELECT * FROM song s
JOIN album a ON a.id = s.album_id
WHERE LOWER(CONCAT_WS(' ', album.name, song.name) = LOWER('Meteora Breaking The Habit')

I wish to create an index on the above type of search terms but it looks like I can't have an index across multiple tables. In this case, would it make sense to de-normalize the table and put the song & album name in a single table or is there a better approach?

Reference Table:

CREATE TABLE album (
   id SERIAL PRIMARY KEY,
   name text NOT NULL
);

CREATE TABLE song (
   id SERIAL PRIMARY KEY,
   name text NOT NULL,
   album_id integer NOT NULL,
   CONSTRAINT album_id_fk FOREIGN KEY (album_id) REFERENCES album (id)
);

P.S. For the sake of understanding, I'm ignoring the fact that query can be song name first & album name later. Also ignoring that query can contain band name too. The query sent by the client is of the form album.name + ' ' + song.name.

Best Answer

If you are getting the LOWER('Meteora Breaking The Habit') portion of the query from a source that it outside your control, then I would recommend keeping your song/album table structure (although remove album_id from song table), but include a 3rd table:

CREATE TABLE ALBUM_SONG (
  album_id,
  song_id,
  album_song_name -- this would be album.name || song.name
)

This is essentially a join table between the two for M-M relationships ('cause hey - the same song could be on multiple albums)..

Then you can index and query the join table on LOWER(album_song_name)

Related Solutions

Postgresql – Postgres multiple joins slow query, how to store default child record

You write:

Each customer can have multiple sites, but only one should be displayed in this list.

Yet, your query retrieves all rows. That would be a point to optimize. But you also do not define which site is to be picked.

Either way, it does not matter much here. Your EXPLAIN shows only 5026 rows for the site scan (5018 for the customer scan). So hardly any customer actually has more than one site. Did you ANALYZE your tables before running EXPLAIN?

From the numbers I see in your EXPLAIN, indexes will give you nothing for this query. Sequential table scans will be the fastest possible way. Half a second is rather slow for 5000 rows, though. Maybe your database needs some general performance tuning?

Maybe the query itself is faster, but "half a second" includes network transfer? EXPLAIN ANALYZE would tell us more.

If this query is your bottleneck, I would suggest you implement a materialized view.

After you provided more information I find that my diagnosis pretty much holds.

The query itself needs 27 ms. Not much of a problem there. "Half a second" was the kind of misunderstanding I had suspected. The slow part is the network transfer (plus ssh encoding / decoding, possibly rendering). You should only retrieve 100 rows, that would solve most of it, even if it means to execute the whole query every time.

If you go the route with a materialized view like I proposed you could add a serial number without gaps to the table plus index on it - by adding a column row_number() OVER (<your sort citeria here>) AS mv_id.

Then you can query:

SELECT *
FROM   materialized_view
WHERE  mv_id >= 2700
AND    mv_id <  2800;

This will perform very fast. LIMIT / OFFSET cannot compete, that needs to compute the whole table before it can sort and pick 100 rows.

pgAdmin timing

When you execute a query from the query tool, the message pane shows something like:

Total query runtime: 62 ms.

And the status line shows the same time. I quote pgAdmin help about that:

The status line will show how long the last query took to complete. If a dataset was returned, not only the elapsed time for server execution is displayed, but also the time to retrieve the data from the server to the Data Output page.

If you want to see the time on the server you need to use SQL EXPLAIN ANALYZE or the built in Shift + F7keyboard shortcut or Query -> Explain analyze. Then, at the bottom of the explain output you get something like this:

Total runtime: 0.269 ms

Postgresql – postgres composite index design

I would try making two composite partial indexes like

CREATE INDEX idx1 ON tw_schedules (scenario_id, type_well_id) WHERE type_well_id IS NOT NULL;

CREATE INDEX idx2 ON tw_schedules (scenario_id, tw_import_id) WHERE tw_import_id IS NOT NULL;

But, of course, everything depends on the selectivity of these indexes.

Best Answer

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Postgresql – Postgres multiple joins slow query, how to store default child record

pgAdmin timing

Postgresql – postgres composite index design

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