PostgreSQL – Performing Multiple Counts Within a Join

postgresql

I am modeling tokens extracted from tweets and have the below table structure:

CREATE TABLE process (
        id SERIAL PRIMARY KEY,
        ...
);

CREATE TABLE tweet (
        id SERIAL PRIMARY KEY,
        process_id INTEGER NOT NULL REFERENCES process(id) ON DELETE CASCADE ON UPDATE CASCADE,
        cleaned_text TEXT,
        process_tweet_id INTEGER NOT NULL,
        constraint tbl_statusid_processid unique(status_id, process_id)
        ...
);

CREATE TABLE token (
    id SERIAL PRIMARY KEY,
    tweet_id INTEGER NOT NULL REFERENCES tweet(id) ON DELETE CASCADE ON UPDATE CASCADE,
    type TEXT,
    value TEXT,
    ...
);

The token table has a foreign key for the tweet table and the tweet table has a foreign key for the process table.

The 'type' field in the token table can be 1 of 7 or 8 possible values (e.g. 'LOCATION', 'PERSON', etc.). The possible types are static and enforced elsewhere within the application, so there is no chance of any diversion from those set types.

I am trying to create a query that will return all of the tweets referencing a given process_id, the total number of tokens referencing each tweet as well as the number of each TYPE of token referencing each tweet (each type having its own column in the result set).

  id   |     status_id      | tokencount | loctoken | persontoken 
-------+--------------------+------------+----------+-------------
  6158 | 672636476602122241 |          3 |        2 |           1
 10295 | 674183390430552064 |          4 |        2 |           2

The above result set is basically what I am hoping to get back. So, tweet ID# 6158 has a total of 3 tokens, 2 of which are location tokens and 1 is a person token. Tweet ID# 10295 has a total of 4 tokens, 2 locations and 2 persons.

What I have so far is a simple JOIN, but I am unsure how to include the count for each type of token per tweet…

SELECT      tweet.id, tweet.status_id, count(token.id) AS tokenCount    
FROM        tweet     
INNER JOIN  token on token.tweet_id = tweet.id     
WHERE       tweet.process_id = 5     
GROUP BY    tweet.id;

I thought I could simply add count(token.type = 'LOCATION') like the below, but that simply repeated the count(token.id) column.

SELECT      tweet.id, tweet.status_id, count(token.id) AS tokenCount, count(token.type = 'LOCATION) AS locToken, count(token.type = 'PERSON') AS personToken    
FROM        tweet     
INNER JOIN  token on token.tweet_id = tweet.id     
WHERE       tweet.process_id = 5     
GROUP BY    tweet.id;

Can anyone provide the query that would return what I am looking for? Hopefully with a good explanation of the solution?

I would also really like to know why count(token.type = 'LOCATION') did not work and simply returned the same number as count(token.id). I tried it on a whim and really thought I was just going to get an error.

Best Answer

You were nearly there, you can do this with case when ... end

SELECT      tweet.id, tweet.status_id, count(token.id) AS tokenCount, 
sum(case when token.type = 'LOCATION' then 1 else 0 end) AS locToken, 
sum(case when token.type = 'PERSON' then 1 else 0 end) AS personToken    
FROM        tweet     
INNER JOIN  token on token.tweet_id = tweet.id     
WHERE       tweet.process_id = 5     
GROUP BY    tweet.id;

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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 – Possible to have nested inserts in Postgres 8.4

You should be able to do something like this with a writable CTE:

WITH i AS (
   INSERT INTO host (hostname, hostrole) VALUES ('foobar', 'Virtual') RETURNING id
)
INSERT INTO interface (name, mac, host)
SELECT 'eth0', '00:50:56:9d:34:d4', id
FROM i

(untested, but it should be something like that)

Writable CTE is in PostgreSQL 9.1 and up.

Best Answer

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