PostgreSQL – Selecting Single Row Based on Condition

postgresql

Consider the following table structure

Table Structure

 Table "public.studenttable"
 Column  |  Type   | Modifiers 
---------+---------+-----------
 id      | integer | not null
 uid     | text    | not null
 subject | text    | 
Indexes:
    "studenttable_pkey" PRIMARY KEY, btree (id, uid)

select * from studenttable

 id | uid | subject 
----+-----+---------
  1 | S01 | Maths
  1 | S02 | Science
  1 | S03 | English
  2 | S02 | English

My objective is to get rows which have only English as a subject which should return id = 2 row only. I have following approaches in mind

1) Return subjects as a string separated by semicolon for particular id and run a loop to match violations.

select id,(array_agg(subject),';') 
from studenttable 
group by id

2) The second approach is to do a self join on studenttable to make sure that id = 1 has only one subject.

I am not able to form the final query. Also is there any alternative?

Best Answer

Assuming that the same id cannot have the same subject more than once.

SELECT *
FROM   studenttable s
WHERE  subject = 'English' 
AND    NOT EXISTS (
   SELECT FROM studenttable s1
   WHERE s1.id = s.id
   AND   s1.uid <> s.uid  -- exclude self
   );

Find all rows with subject='English' where no other row for the same id exists.

The query is fast as it can use the index backing your PK. We could also use

AND   s1.subject <> 'English'

AND   s1.subject <> s.subject

To allow duplicate entries for 'English', but that would be slower without index support.

How do I remove duplicate records in a join table in PostgreSQL?

Better Queries

This is just your query reformatted with aliases and some noise removed to clear the fog:

SELECT count(DISTINCT t.id)
FROM   tickets      t
JOIN   transactions tr ON tr.objectid = t.id
JOIN   attachments  a  ON a.transactionid = tr.id
WHERE  t.status <> 'deleted'
AND    t.type = 'ticket'
AND    t.effectiveid = t.id
AND    tr.objecttype = 'RT::Ticket'
AND    a.contentindex @@ plainto_tsquery('frobnicate');

Most of the problem with your query lies in the first two tables tickets and transactions, which are missing from the question. I'm filling in with educated guesses.

t.status, t.objecttype and tr.objecttype should probably not be text, but enum or possibly some very small value referencing a look-up table.

`EXISTS` semi-join

Assuming tickets.id is the primary key, this rewritten form should be much cheaper:

SELECT count(*)
FROM   tickets t
WHERE  status <> 'deleted'
AND    type = 'ticket'
AND    effectiveid = id
AND    EXISTS (
   SELECT 1
   FROM   transactions tr
   JOIN   attachments  a ON a.transactionid = tr.id
   WHERE  tr.objectid = t.id
   AND    tr.objecttype = 'RT::Ticket'
   AND    a.contentindex @@ plainto_tsquery('frobnicate')
   );

Instead of multiplying rows with two 1:n joins, only to collapse multiple matches in the end with count(DISTINCT id), use an EXISTS semi-join, which can stop looking further as soon as the first match is found and at the same time obsoletes the final DISTINCT step. Per documentation:

The subquery will generally only be executed long enough to determine whether at least one row is returned, not all the way to completion.

Effectiveness depends on how many transactions per ticket and attachments per transaction there are.

Determine order of joins with `join_collapse_limit`

If you know that your search term for attachments.contentindex is very selective - more selective than other conditions in the query (which is probably the case for 'frobnicate', but not for 'problem'), you can force the sequence of joins. The query planner can hardly judge selectiveness of particular words, except for the most common ones. Per documentation:

join_collapse_limit (integer)

[...]
Because the query planner does not always choose the optimal join order, advanced users can elect to temporarily set this variable to 1, and then specify the join order they desire explicitly.

Use SET LOCAL for the purpose to only set it for the current transaction.

BEGIN;
SET LOCAL join_collapse_limit = 1;

SELECT count(DISTINCT t.id)
FROM   attachments  a                              -- 1st
JOIN   transactions tr ON tr.id = a.transactionid  -- 2nd
JOIN   tickets      t  ON t.id = tr.objectid       -- 3rd
WHERE  t.status <> 'deleted'
AND    t.type = 'ticket'
AND    t.effectiveid = t.id
AND    tr.objecttype = 'RT::Ticket'
AND    a.contentindex @@ plainto_tsquery('frobnicate');

ROLLBACK; -- or COMMIT;

The order of WHERE conditions is always irrelevant. Only the order of joins is relevant here.

Or use a CTE like @jjanes explains in "Option 2". for a similar effect.

Indexes

B-tree indexes

Take all conditions on tickets that are used identically with most queries and create a partial index on tickets:

CREATE INDEX tickets_partial_idx
ON tickets(id)
WHERE  status <> 'deleted'
AND    type = 'ticket'
AND    effectiveid = id;

If one of the conditions is variable, drop it from the WHERE condition and prepend the column as index column instead.

Another one on transactions:

CREATE INDEX transactions_partial_idx
ON transactions(objecttype, objectid, id)

The third column is just to enable index-only scans.

Also, since you have this composite index with two integer columns on attachments:

"attachments3" btree (parent, transactionid)

This additional index is a complete waste, delete it:

"attachments1" btree (parent)

Details:

Is a composite index also good for queries on the first field?

GIN index

Add transactionid to your GIN index to make it a lot more effective. This may be another silver bullet, because it potentially allows index-only scans, eliminating visits to the big table completely.
You need additional operator classes provided by the additional module btree_gin. Detailed instructions:

Inner join using an array column

"contentindex_idx" gin (transactionid, contentindex)

4 bytes from an integer column don't make the index much bigger. Also, fortunately for you, GIN indexes are different from B-tree indexes in a crucial aspect. Per documentation:

A multicolumn GIN index can be used with query conditions that involve any subset of the index's columns. Unlike B-tree or GiST, index search effectiveness is the same regardless of which index column(s) the query conditions use.

Bold emphasis mine. So you just need the one (big and somewhat costly) GIN index.

Table definition

Move the integer not null columns to the front. This has a couple of minor positive effects on storage and performance. Saves 4 - 8 bytes per row in this case.

                      Table "public.attachments"
         Column      |            Type             |         Modifiers
    -----------------+-----------------------------+------------------------------
     id              | integer                     | not null default nextval('...
     transactionid   | integer                     | not null
     parent          | integer                     | not null default 0
     creator         | integer                     | not null default 0  -- !
     created         | timestamp                   |                     -- !
     messageid       | character varying(160)      |
     subject         | character varying(255)      |
     filename        | character varying(255)      |
     contenttype     | character varying(80)       |
     contentencoding | character varying(80)       |
     content         | text                        |
     headers         | text                        |
     contentindex    | tsvector                    |

Best Answer

Related Solutions

PostgreSQL – Select and Count Based on a Condition

PostgreSQL – Improve Full-Text Search Performance with Accurate Row Estimates