![]() Let’s proceed with uncovering this mystery.ĭepending on the index selectivity, PostgreSQL utilizes one of two methods. It is critically important to fully understand how the indexes bitmapping works in PostgreSQL, especially because other major DBMS do not have similar access methods, documentation does not go into many details about it, and there is a lot of confusion among users regarding how it works. Each time I promised “to talk about it later,” and now this “later” is here. It has been several times already that the term ‘bitmap’ appeared in execution plans. Execution plan with sequential scan on a small table What does “bitmap” mean? Index Cond : ( departure_airport = 'JFK' :: bpchar )įigure 1. ![]() The flight table in the postgres_air database has a foreign key constraint on the aircraft_code : To illustrate what “reasonable” means in this context, let’s consider the following example. We remember that only indexes with reasonably low selectivity are useful. To make sure that deletion wouldn’t violate any of these integrity constraints, Postgres had to sequentially scan thirteen large tables.ĭoes it mean that we always should create an index on column(s) which has a foreign key constraint? Not necessarily. It turned out that the table was a parent table for thirteen foreign key constraints, and none of them was supported by an index. I observed a user wondering why a deletion of one row (supported by a unique index) took over 10 minutes. This misconception often backfires when we need to delete a row from a parent table. The column(s) in the parent table must have a primary key or unique constraint on them, however, nothing is automatically created on a child table. This is not true.Ī foreign key is a referential integrity constraint it guarantees that for each non-null value in the child table (i.e., the table with the foreign key constraint), there is a matching unique value in the parent table (i.e., the table it is referencing). ![]() What about foreign keys? Do they automatically create any indexes? A common misconception is the belief that the presence of a foreign key necessarily implies the presence of an index on the child table. Note: you do not need to create a unique constraint to create a unique index: Note: If you want to try the code, the appendix has the information you need to install the sample database we will be using, as well as some necessary updates before starting with the current article.įor example, the table booking has a primary key on booking_id and a unique key on booking_ref. Any non-null unique set of columns can be chosen to be a primary key for a table thus, there is no programmatic way to determine the best candidate for a table’s primary key. ![]() In contrast, UNIQUE constraints in PostgreSQL allow for NULL values.Ī table can only have a single primary key (though a primary key is not required), but can also have multiple unique constraints. It is possible to define a primary key as a combination of several attributes it just has to satisfy two conditions: the combination must be UNIQUE and NOT NULL for all participating attributes. Although it often helps to have a numeric incremental primary key (which in this case is called a surrogate key), a primary key does not have to be numeric, and moreover, it does not have to be a single-attribute constraint. Is there any difference between a primary key and a unique constraint? Many SQL developers assume that a primary key must be an incrementing numeric value and that each table “has” to have a primary key. PostgreSQL automatically creates a unique index to support any primary key or unique constraint on a table. The definition of a unique index states just that: an index is unique if for each indexed value there is exactly one matching row in the table. The smallest number of rows is one, thereby, the most useful indexes are unique indexes. In the previous article we learned that the most helpful indexes are indexes with the lowest selectivity, which means that each distinct value in an index corresponds to a small number of rows. We will learn whether (and how) database constraints and indexes are related (or not), how exactly index bitmap scan works, and explore some additional index options available in PostgreSQL. But there is much more to learn about indexes! In this blog, we will keep exploring B-tree indexes. In that article, we learned what an index is, and how exactly indexes can help with query execution. In this blog, we continue our exploration on PostgreSQL indexes which we started here.
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