What causes slow query performance in database development?

In my experience, the most common causes of database performance issues are: * Incorrect data types (VARCHAR instead of DATE, BIGINT for year or e-mail as VARCHAR(4000)) * Missing or incorrect indexes * Saving files in the database (Rather, you could use the connection between the database entry and the location of the corresponding file in the file system. If necessary, you can also use additional hash calculation to check the integrity of the files)

In my opinion, one of the worst cause of slow query performance in RDBMS is: * You should avoid using MAX, 8000 and 4000(for only N') length for string data types. RDBMS' always store each of them in different 8KB pages although it can include 1 byte of data for a column of a row. Therefore, always a performance issue during DML operations. By avoiding this, you can store thousands of them in a 8KB page instead of a single row or column.

* Always you should avoid using "*(star)" in SELECT statements because it will extra I/O operation to retrieve unnecessary data. * Most performance issues arise from Non-SARGable queries. (Use index correctly and make it high-selective) * Better think about where predicates should be used according to your needs. (Using predicates in different clauses such as JOIN, WHERE, HAVING may cause performance problems because they will cause different operations.)

* Always use parameterized queries to store right query plans and analyze them in the future. (Parameterized queries always generate single query plan for same queries) * Analyze query plans and execution statistics.

Incorrect Db configuration settings such as buffer pool size, cache settings or thread pool size. Adjust setting based on workload and requirements.

Default database settings are always the worst. * First, if you don't know anything about configurations find a benchmark or stress software and get result. After that, you can research what should you do. * Research best practices to configure database options.

Since Db's volume increases with time, queries take longer to execute. we need to do data archiving, partitioning and purging.

I recommend a few things for single database architectures below: * Think and discuss column's data types of all tables to reduce storage issues. * For tables larger than 512MB or 1024MB, consider using physical files. (You can use these files in different disks and reduce I/O operations) * Consider using table partitioning for your time-series tables. * Use archiving for old datas and put them into slower disks if you rarely use them.

The physical storage configuration of the Db and i refer to disk speed and RAID settings. Ensure storage system meets the requirements of DB workload.

Always use correct data types and avoid redundant extra precision, scale and length. Try to use physical files like file groups for the larger tables and distribute them into different disks. Consider using table partitioning and archiving