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 + F7
keyboard shortcut or Query -> Explain analyze
. Then, at the bottom of the explain output you get something like this:
Total runtime: 0.269 ms
The error has occurred because you are referencing a column in another table which is not unique. The good answers are already given by Lennart and Balazs Papp.
I would like to explain why do we need a unique column in the parent table. As you said you want to keep duplicate values in the column used for the foreign key which is not possible while creating the table. But you can create a reference to an existing table which contains duplicate values.
If you create a primary key with non-unique index and NOVALIDATE
option then it is possible. BUT this can lead to confusing results.
Let me explain a situation.
I have created a table with one column ID
which has a primary key constraint with a non-unique index.
SQL>CREATE TABLE t1(id NUMBER);
SQL>CREATE INDEX t1_index on t1(id);
SQL>INSERT INTO t1 VALUES(1);
SQL>INSERT INTO t1 VALUES(1);
SQL>COMMIT;
SQL>SELECT id FROM t1;
ID
----------
1
1
SQL>ALTER TABLE t1 ADD CONSTRAINT t1_pk PRIMARY KEY (id) USING INDEX t1_index NOVALIDATE;
Let's create another table to reference the first table.
SQL>CREATE TABLE t2(id NUMBER, CONSTRAINT t2_fk FOREIGN KEY(id) REFERENCES t1(id));
Generate some records.
SQL>INSERT INTO t2 VALUES(1);
SQL>COMMIT;
Table t2
has a value 1
which is referencing parent table t1
which has a duplicate value of 1
. Which one the child table's id
will refer to?
In the above scenario, the foreign key works fine
but the primary key in table t1
works only for new values.
Conclusion:
A foreign key must always refer to a column or columns declared as either PRIMARY KEY or UNIQUE in Oracle.
Best Answer
You need to add super-key (unique index/constraint) on both (id,topic_id). This gives you the "target" uniqueness to create your foreign key. This acts like a CHECK constraint in this case.
Note: id remains as primary key to preserve the model. Even if id is serial, it's would be wrong from a modelling perspective to change the PK to (id,topic_id)