Postgresql – Convert a lookup value to foreign key when a query is run

foreign keyinsertpostgresql

Currently we have a legacy system running queries on a Postgres server to update/insert records. One of the obstacles we are having is that we have to convert text values to the correct foreign key for a lookup table. Please see below for a better explanation:

Query:

INSERT INTO public.people(name, state)
VALUES ('John', 'CA');

Tables:

public.people

CREATE TABLE people
(
  name text,
  state smallint,
  CONSTRAINT state_key FOREIGN KEY (state)
      REFERENCES states (states_id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION
)
WITH (
  OIDS=FALSE
);

CREATE INDEX fki_state_key
  ON people
  USING btree
  (state);

public.states

CREATE TABLE states
(
  states_id smallint,
  name text,
  CONSTRAINT state_unique UNIQUE (states_id)
)
WITH (
  OIDS=FALSE
);

What is the best way of converting 'CA' to the associated ID in the public.states table? The particular interface we are using to talk to Postgres is called Postgrest (https://github.com/begriffs/postgrest) and would require additional SELECT statements instead of a subqueries. We want minimize the amount of calls as much as possible.

Also, on the legacy system that is using that interface it does not use a lookup table, which makes it difficult to determine which fields need to be converted for a lookup table.

Is there a native way in Postgres to handle these types of conversions? If you need additional information please let me know.

"PostgreSQL 9.4.5 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.8.3 20140911 (Red Hat 4.8.3-9), 64-bit"

Best Answer

There are various ways, depending on the use case. For your example:

INSERT INTO public.people(name, states_id)
VALUES ('John', (SELECT states_id from public.states WHERE name = 'CA'));

Or:

INSERT INTO public.people(name, states_id)
SELECT 'John', states_id
FROM   public.states
WHERE  name = 'CA';

Assuming public.states.name to be unique - there should be a constraint in the table definition.
And a column public.people.states_id, obviously.

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"I'm certain every time that I'm bringing a gun to a knife fight, but my googling has been fruitless." What makes you think that? RDBMSs are built to 'join'

If your aim is to make you SQL more readable then you might prefer the 'using' syntax:

SELECT * FROM candies JOIN colors USING(color_code) WHERE color_value = 'blue';

alternatively, seeing as you are hard-coding 'blue' into your query, you could consider hard-coding the code instead:

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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?

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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.

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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

Best Answer

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