Postgresql – Postgres – Importing multiple rows with Foreign key constraint

foreign keyinsertpgadminpostgresql

I can't seem to find an answer for my problem, maybe I'm not asking the right question but here it goes:

I have basically two tables in pgAdmin, let's say student and grades, eventually with a foreign key constraint (student_id referencing student(id)

CREATE TABLE student (
    id CHAR(2),
    name CHAR(4),
PRIMARY KEY (id));

CREATE TABLE grade (
    student_id CHAR(2),
    grade INT,
FOREIGN KEY (student_id) REFERENCES student(id));

What I'd like to do is copy/import two files into the databases. All good for students but my grade.csv files will contain non-existing student IDs and I want the import to ignore and not insert them. Instead of that, the command fails, nothing is imported.

What would be a correct and efficient way to do this?

Best Answer

The solution to this problem is the one that one uses quite often when importing data: to use a staging table.

Let's say you have a CSV with two fields (student_id and grade). This maps to the grade table nicely, and the only problem is the nonexistent students. So, first create a temporary table and then import the data there:

CREATE TEMPORARY TABLE tmp_grade AS 
    SELECT * 
      FROM grade WHERE FALSE;

Then you have multiple options, but the gist of it is only picking the rows from the temp table that have a matching student:

INSERT INTO grade
SELECT * 
  FROM tmp_grade
 WHERE EXISTS (SELECT 1 FROM student WHERE id = student_id);

And done. The performance of this depends on the size of the data (not surprisingly). The definitions of the unique student.id suggests this won't be the case. However, if it happens, you can still play around with other variations to the same theme.

Notes:

choosing char(2) as the student ID is interesting.
pgAdmin is a client to PostgreSQL, it does not have tables by itself.

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 – Create table with foreign key constraint too slow

Your query got locked waiting for something - my bet is that it was waiting for some other transaction to finish.

When you're doing the create table, just issue (in another psql session):

select * from pg_locks where pid = XXX and not granted;

where xxx is pid of backend doing the create table.

This will show you what is the lock that the create table is waiting for.

Best Answer

Related Solutions

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

pgAdmin timing

Postgresql – Postgres – Create table with foreign key constraint too slow

Related Question