Query routing means taking (part of) the query and letting the worker node that stores the relevant shards handle the query, which is possible when the query filters and joins by the distribution column. Since the data pertaining to a customer is stored on a singular worker node, the MERGE command uses query routing. order_id WHEN NOT MATCHED THEN - New entry, record it in target INSERT ( customer_id, last_order_id, order_center, order_count, last_order ) VALUES ( customer_id, s. order_center = 'New York' THEN - Remove the customer in target DELETE WHEN MATCHED THEN - Existing customer, update the order count UPDATE SET order_count = t. MERGE INTO customers t USING orders s ON ( t. From theory to practice: Diving into use case(s) Let's start by looking at a simple query-routing scenario Note: Each output row of the join can activate at most one when-clause. The corresponding action is then applied, and processing continues for the next row.WHEN clauses are evaluated in the exact specified order until one of them is activated.The first clause that evaluates to true is the one (and the only one) that gets executed. The status of MATCHED or NOT MATCHED is assigned to each row just once, after which the WHEN clauses are evaluated in the order they are written. How MERGE works in Postgres (as of Postgres 15) Figure 1: MERGE command performs a left outer join between the source-table and target-table. Batch operations: When performing bulk operations that may involve both insertions and updates, using MERGE can be more efficient than issuing separate insert and update commands, as it minimizes the number of total commands sent and processed.Real-time data update: If you're dealing with real-time data that's constantly being updated, such as stock prices or sensor readings, you can use MERGE to ensure that the most recent data is always available, without the need to distinguish between new and existing data.Logging: In cases where you're logging events to a table and you want to count occurrences of events, and an event occurs that's already in the table, you increment a counter otherwise, you insert a new row.Data deduplication: If you're handling data that might contain duplicates, and a duplicate row is detected, you can simply update the existing record.MERGE ensures that the database remains consistent with the external data source. Data synchronization: If you're periodically syncing data from an external system and want to update existing records while creating new ones as necessary.Real-world applications of the Postgres MERGE SQL operation You are eager to unlock the full potential of MERGE SQL.From theory to practice: Diving into use case(s).How MERGE works in Postgres (as of Postgres 15).Real-world applications of the Postgres MERGE SQL operation.It will delve into some of the practical use-cases, and subsequently elaborate on the different strategies employed by Citus for handling MERGE in a distributed environment. In this blog post, you will learn a high-level overview of the functioning of Postgres MERGE. MERGE provides a single SQL statement that can conditionally INSERT, UPDATE or DELETE rows, a task that would otherwise require multiple procedural language statements, using INSERT with ON CONFLICT clause etc. If it's not for some reason to do with SQL's deeper architecture, I'd like to understand why.Postgres community released a new feature, in Postgres 15.0, that performs actions to modify rows in the target table, using the data from a source. This feels like a more correct & less wasteful way to do it. In the past I've done operations like this by doing two SELECTS, one of which is against the item_templates table, the other against the unique_items table, and taken their UNION. Or if this is something you can't do conceptually - have each row choose its own join - I'd love to get a link to a deeper explanation. If there's a quick fix for the syntax, I'd love to hear it. This is the sort of thing I've been trying, without success: SELECT item_table, item_id, * So when I find a record of an item in the item_instances table, I want to look up more information about it, and I need to pull that info from the correct table. They are now stored in the unique_items table. Say I've got a table of item_instances in my game, of all the items in the game world, and some of the items are cookie-cutter copies of generic items from the item_templates table, and other items started off as templates, and then acquired unique properties as players used them. I'm attempting to use a CASE expression to select which table each row should be joined with.
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