sqlquerypp: SQL query preprocessor

sqlquerypp is a library for preprocessing SQL queries. Main purpose is writing highly optimized queries with a simplified syntax, allowing for both maintainability and high performance.

Limitations

Currently, only MySQL 8.4 syntax is supported.

Why preprocess SQL queries?

SQL (Structed Query Language) follows a declarative paradigm, i.e. a query explains "what should be done" not "how should it be done". This stands in contrast to imperative programming, which expresses the "how should a certain task be fulfilled" aspect.

Database systems' internals are responsible for maintaining this aspect. However, for certain and large data structures, writing down "naive" queries sometimes result in poor performance.

Supported performance optimizations

Combined UNION queries

Consider the following original query:

SELECT entity_b.*
FROM entity_b
INNER JOIN entity_a
  ON entity_a.id = entity_b.entity_a_id
  AND entity_a.criteria = 1337;

This is a very simplified example, but if you assume entity_b contains a multitude of items, even correct index conditions may exhaust any DBMS' join buffer.

If network overhead is acceptable, a fitting alternative approach could be a loop on the application side (Python pseudocode):

all_matches_in_entity_b = []
for entity_a_id in [rec.id
                    for rec in mysql_query("SELECT id FROM entity_a "
                                           "WHERE criteria = 1337")]:
    inner_result = mysql_query("SELECT * FROM entity_b "
                               f"WHERE entity_a_id = {entity_a_id}")
    all_matches_in_entity_b += inner_result

The following statement, which is invalid SQL, translates to a MySQL native construct of Recursive Common Table Expression and UNION fragments when compiled by sqlquerypp. This allows for maximal query performance, because the inner query with reduced complexity is still taken into account. At the same time, it grants minimal I/O overhead as only one query is executed on the database:

combined_result (SELECT id FROM entity_a WHERE criteria = 1337) AS $id {
    SELECT * FROM entity_b WHERE entity_a_id = $id;
}