PostgreSQL Cheat Sheet

Written — Updated

Also see PostgreSQL CLI Cheat Sheet.

Triggers 🔗

Full Documentation

Here’s an example that updates a row’s timestamp whenever a query updates the row without changing the timestamp.

CREATE FUNCTION update_timestamp()
    NEW.updated = now();
$$ LANGUAGE 'plpgsql';

CREATE TRIGGER update_timestamp_trigger BEFORE UPDATE
    ON the_table
    WHEN (NEW.updated IS NOT DISTINCT FROM OLD.updated
       OR NEW.updated IS NULL)
    EXECUTE FUNCTION update_timestamp();

The operation can be one of UPDATE, INSERT, OR DELETE. Triggers can execute BEFORE or AFTER the operation, and can run FOR EACH ROW, or once for the query using FOR EACH STATEMENT.

In a function called per row, the function can reference the original and new row values as OLD and NEW. The values can be modified directly in the function, and then the row returned by the function will become the new value.

Indexing and matching on JSON values inside an array 🔗

Imagine you have a JSONB field that is an array of objects, and you want to match on rows where any value of the array fits your criteria. In my particular case, it looked similar to [{"matches": [1, 2, 3], ...otherData }, {"matches": [2, 3, 4], ...otherData}].

Fortunately, PostgreSQL’s JSON support is top-notch and this is actually very easy. The A @> B operator can check if value A “contains” value B. For arrays, you can think of this as more of an overlap than strict containment. If your array criteria has more than one item, they are checked individually against the source data and each one must match.

-- At least one item in the array overlaps with the criteria, so
-- this is true.
select '[{"a": [2, 3, 4]}, {"a": [1, 2, 3]}]'::jsonb @>
       '[{"a": [3, 4]}]'::jsonb as matches;

-- When matching against arrays, the match criteria must also be
-- an array. Again, think overlap more than containment.
select '[{"a": [2, 3, 4]}, {"a": [1, 2, 3]}]'::jsonb @>
       '{"a": [3, 4]}'::jsonb as matches;

-- Order of items in the array does not matter.
select '[{"a": [2, 3, 4]}, {"a": [1, 2, 3]}]'::jsonb @>
       '[{"a": [4, 3]}]'::jsonb as matches;

-- Here we match against an array item where the array doesn't
-- overlap, so it's false.
select '[{"a": [2, 3, 4]}, {"a": [1, 2, 3]}]'::jsonb @>
       '[{"a": [4, 5]}]'::jsonb as matches;

-- Array items are matched individually, so here there are
-- two array items [4] and they both overlap with the same
-- array [2,3,4]. Postgres calls this a match.
select '[{"a": [2, 3, 4]}, {"a": [1, 2, 3]}]'::jsonb @>
       '[{"a": [4]}, {"a": [4]}]'::jsonb as matches;

-- And here one of the array items matches but the other
-- does not, so it's false.
select '[{"a": [2, 3, 4]}, {"a": [1, 2, 3]}]'::jsonb @>
       '[{"a": [4]}, {"a": [5]}]'::jsonb as matches;

Finally, to make it fast in a real application, just create a GIN index on the entire column.

CREATE INDEX ON table USING GIN (data_column);

And if you only need the @>, @@, and @? operators, you can specify jsonb_path_ops to make the index faster and smaller.

CREATE INDEX ON table USING GIN (data_column jsonb_path_ops);

The PostgreSQL documentation provides a thorough and easy-to-read discussion of the tradeoffs between the two types of indexes.

First Row per Group 🔗

Sometimes you are grouping by a certain value, and want to get just the first matching row for each group. You can do this by taking the first value of an ordered ARRAY_AGG, but there’s a faster and easier way.

Postgres lets you use DISTINCT ON instead of GROUP BY and order by the other fields you want as well. It’s guaranteed that the DISTINCT ON will pick the first value in the ordering for each row.

Say we have a table of data models and we want to get the latest version for each one. This is all you need:

FROM models
ORDER BY id, model_version DESC

To speed it up, create an ordered index with the same fields used in the order:

CREATE INDEX ON models (id, model_version DESC)

Lateral Joins 🔗

These allow a subquery in a join to reference values from the top-level query.

Most useful when aggregating over things in the subquery to produce an array of results, without needing to group by which is especially inconvenient when aggregating multiple independent joins.

This example from the Zapatos documentation gets a single associated author and multiple associated tags for each book.

SELECT result
  SELECT to_jsonb ("books".*) || jsonb_build_object($1:text, "ljoin_0".result, $2::text, "ljoin_1".result) AS result
  FROM "books"
    SELECT to_jsonb ("authors".*) AS result
    FROM "authors"
    WHERE ("id" = "books"."authorId")
  LIMIT $3) AS "ljoin_0" ON true
    SELECT coalesce(jsonb_agg(result), '[]') AS result
    FROM (
      SELECT to_jsonb ("tags".*) AS result
      FROM "tags"
      WHERE ("bookId" = "books"."id")) AS "sq_tags") AS "ljoin_1" ON true) AS "sq_books"

Thanks for reading! If you have any questions or comments, please send me a note on Twitter.