8/6/2023 0 Comments Postgresql cross join![]() ![]() Nobody likes 30-line SQL queries, so let’s break this down into pieces. But, in 9.3, we can use a lateral join to compute this in one efficient query, with no extensions or PL/pgSQL. If we were using an older version of PostgreSQL, we might write some custom functions in PL/pgSQL, PostgreSQL’s builtin procedural language. We’ll assume that we’ve instrumented our frontend to log events along this flow and that all of the data lives in the event table specified above.*Īs an initial question, let’s figure out how many people view our homepage and what percentage of them enter a credit card within two weeks of that initial homepage view. The first step is to figure out where we’re losing users in our conversion funnel.Īn example conversion funnel between four steps in a signup flow. At Heap, these properties might include the DOM hierarchy of a click, the window title, the session referrer, and so forth.Let’s say we want to optimize our landing page to increase signups. What can we do with this?Ĭonsider a table of click events with the following schema:Įach event is associated with a user and has an ID, a timestamp, and a JSON blob with the event’s properties. Loosely, it means that a LATERAL join is like a SQL foreach loop, in which PostgreSQL will iterate over each row in a result set and evaluate a subquery using that row as a parameter. This is repeated for each row or set of rows from the column source table(s). The resulting row(s) are joined as usual with the rows they were computed from. When a FROM item contains LATERAL cross-references, evaluation proceeds as follows: for each row of the FROM item providing the cross-referenced column(s), or set of rows of multiple FROM items providing the columns, the LATERAL item is evaluated using that row or row set’s values of the columns. (Without LATERAL, each sub- SELECT is evaluated independently and so cannot cross-reference any other FROM item.) This allows the sub- SELECT to refer to columns of FROM items that appear before it in the FROM list. The LATERAL key word can precede a sub- SELECT FROM item. The best description in the documentation comes at the bottom of the list of FROM clause options : Interested in learning more about Heap Engineering? Meet our team to get a feel for what it’s like to work at Heap! What is a LATERAL join? ![]() In this post, I’ll walk through a conversion funnel analysis that wouldn’t be possible in PostgreSQL 9.2. PostgreSQL 9.3 has a new join type! Lateral joins arrived without a lot of fanfare, but they enable some powerful new queries that were previously only tractable with procedural code. start_date ) as days_diffĬROSS JOIN LATERAL ( SELECT to_date (l. In the following example, start_timestamp and end_timestamp are being parsed to a date and the output of those calculations are then being used multiple times from the SELECT statement. Otherwise, you would have to recalculate values for each usage of them in the SELECT statement. Since you can reference columns from other records in the query, you can use LATERAL to calculate values and then reuse them in the main SELECT statement. This is a non-obvious use of LATERAL but one I use often. pl_percentageĬROSS JOIN LATERAL get_trade_pl (t. If you have a User-Defined function that needs to be run for each row of a query, you can use the LATERAL join to call it and return multiple values. Effectively, it behaves like a LEFT JOIN.Ģ. NOTE: I use CROSS JOIN LATERAL above which is eqivalent to LEFT JOIN LATERAL (.) a ON true (as shown in LATERAL examples elsewhere) but I find it more readable. amountĬROSS JOIN LATERAL ( SELECT id as order_id, date, amount Without a LATERAL join this type of query would be non-trivial. Examplesįor each customer, we will return the 2 most recent orders. ![]() It is similar to a correlated subquery in that it can reference values from another query but has the added advantages that it can return multiple values and be used in the FROM clause. In a gist, it allows you to perform a sub-query in the FROM clause and reference column values from other records in the query. (Without LATERAL, each sub-SELECT is evaluated independently and so cannot cross-reference any other FROM item.) This allows the sub-SELECT to refer to columns of FROM items that appear before it in the FROM list. The LATERAL key word can precede a sub-SELECT FROM item. So, what is a LATERAL join anyway? From the PostgreSQL documentation: The LATERAL join is an interesting and powerful join type that is a bit intimidating at first but when you take a closer look it is very useful in certain scenarios. ![]()
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