why does union query take far longer than the individual sub queries

AI generated answer (using internal Slack threads):

A union query can take longer than individual subqueries due to several reasons: 1. Data Volume: Union queries combine the results of two or more select statements into a single result set. If each of these select statements is returning a large amount of data, the union operation can be quite expensive in terms of time and resources. 2. Complexity: Union queries can be more complex than individual queries, especially if they involve multiple joins, subqueries, or complex conditions. This added complexity can increase the time it takes to execute the query. 3. Data Conversion: If the data types of the columns being unioned are not the same, the database may need to perform implicit data type conversions. These conversions can be time-consuming, especially if they involve converting text fields to date values or vice versa. 4. Index Usage: Depending on the database system and the specific queries involved, a union query may not be able to make as effective use of indexes as individual queries. This can result in slower performance. 5. Server Load: The performance of a union query can also be affected by the current load on the database server. If the server is under heavy load, it may take longer to execute the union query. To improve the performance of union queries, you could consider the following strategies: - Break down complex union queries into simpler, independent queries that can be run asynchronously and merged afterwards. - Limit the amount of data returned by each select statement in the union query. - Ensure that the data types of the columns being unioned are the same to avoid costly data type conversions. - Optimize the use of indexes in your queries. - Monitor and manage the load on your database server. ****** Disclaimer: This response was generated by an artificial intelligence assistant based on previous conversations from multiple Slack channels, and may contain inaccuracies or mistakes. ******