Postgresql foreign key3/25/2023 The query also filters for either the parent or child table being larger than 10MB. The query tries to filter for the best indexing candidates, but it is just a query and you need to use your judgement on what you know about the tables. Now you're ready to run the query on your own database and look at the results. Also, if the FK is very low cardinality (like, say, only four possible values) then it's probably also a waste of resources to index it. If both tables are small, or if the parent table is small and the FK is used only to prevent bad data entry, then there's no reason to index it. This means most FKs, but not all of them. The child table is large and the FK is used to filter (WHERE clause) records on the child table.The parent table is large and the FK is used for JOINs.The child table is large and the parent table gets updates/deletes.This means that it's important to have an index on the child side of the FK if any of the following are true: If the "child" table is large, this can be substantially speeded up by having an index on the FK column. If there are, it needs to perform the action you have defined (such as CASCADE, SET NULL or RESTRICT). The way it works is this: before letting you delete or change a row in the "parent" table, Postgres has to verify that there are no rows in the "child" table referencing the FK value that might be going away. The second occasion is news to some DBAs. when updating or deleting a row from the "parent" table.when doing JOIN and lookup queries using the FK column.There's two times that indexes on the child side of FKs are used: Now, in order to understand how to interpret this, you have to understand why you would or would not have an index on an FK, and what sort of indexes are valid. Indexes on the "parent", or "referenced" side of the FK are automatically indexed (they have to be, because they need to be unique), so we won't talk about them further. I have added a query for finding foreign keys without indexes to pgx_scripts. But which ones need it? Well, fortunately, you can interrogate the system catalogs and find out. There are some good reasons for this (see below), but it does give users another opportunity to forget something, which is indexing the foreign keys (FKs) that need it. You must use nested select statements to write this query.Ĭ)Write the previous query using join operations.Unlike some other SQL databases, PostgreSQL does not automatically create indexes on the "child" (in formal language "referencing") side of a foreign key. Report the user names of the tweeter and retweeter (the person who retweeted the tweet) as well as the tweet text and date.ī)Find all distinct users (userName and city) who follow a user who follow user “Yoshua Bengio”. (12,'Rhonda Ferguson', 'NY', ' 11:41:26-08') Ī)Find all users who retweeted a tweet that was tagged with “#Iphone” after 2015. CREATE TABLE Users ( userId SERIAL PRIMARY KEY, userName VARCHAR, city VARCHAR, lastLogin TIMESTAMP ) ĬREATE TABLE Tweets ( tweetId SERIAL PRIMARY KEY, userId INT NOT NULL REFERENCES Users ( userId ), tweetTimestamp TIMESTAMP NOT NULL, tweetText VARCHAR ) ĬREATE TABLE Retweets ( userId INT REFERENCES Users ( userId ), tweetId INT REFERENCES Tweets ( tweetId ), retweetTimestamp TIMESTAMP, PRIMARY KEY ( userId, tweetId, retweetTimestamp )) ĬREATE TABLE TweetTags ( tweetId INT, hashtag VARCHAR CHECK ( hashtag LIKE '#%'), PRIMARY KEY ( tweetId, hashtag )) ĬREATE TABLE Follows ( followerId INT REFERENCES Users ( userId ), followeeId INT REFERENCES Users ( userId ), PRIMARY KEY ( followerId, followeeId ))
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