![]() ![]() So we’re probably losing around 21 bytes per row, with the declared types accounting for 59 bytes of actual space, for a total row length of 80 bytes without row overhead. 4 bytes between order_total and order_dt.The next question is: how much of that is padding? Well, here’s an estimate of what minimum we’re wasting based on the declared alignment: Now we can use that as our benchmark for a value we want to beat. Pg_size_pretty(pg_relation_size('user_order')) AS size_pretty SELECT pg_relation_size('user_order') AS size_bytes, ![]() Ship_dt, item_ct, ship_cost, receive_dt, tracking_cd Is_shipped, user_id, order_total, order_dt, order_type, Let’s fill the table with one million rows and check the resulting size: INSERT INTO user_order ( As a result from this particular column arrangement, there’s almost a slight amount of buffering between every single column pair. To avoid this insanity, Postgres pads each smaller column to match the size of the next consecutive column. For now, consider the constant size transitions and the effect that may have on alignment. For NUMERIC and TEXT, things are a bit trickier, but we’ll need to address that later. The typalign column dictates the intended alignment type according to the values listed in the pg_type manual page.įrom earlier discussion, we can see that the INT types trivially map to their respective byte sizes. JOIN pg_attribute a ON (a.attrelid = c.oid) Given that, this is what Postgres sees: SELECT a.attname, t.typname, t.typalign, t.typlen Seeing the id column at the end is a good indicator that column order wasn’t part of the architecture or planning roadmap. It’s not uncommon for column orders to be dictated by a hurried dev simply jotting down attributes, or an ORM that generates output from an arbitrary hashed key position. If it looks a little weird, that’s because it is. Is_shipped BOOLEAN NOT NULL DEFAULT false, But consider a contrived ordering system with this table: CREATE TABLE user_order ( Intensity Intervalsīy itself, this may not necessarily be a problem. Instead of 2 + 8 = 10, our math becomes 8 + 8 = 16. This suggests that an empty Postgres row requires 24 bytes of various header elements, a SMALLINT is 2 bytes, a BIGINT is 8 bytes, and combining them is… 16 bytes? That’s no mistake Postgres is padding the smaller column to match the size of the following column for alignment purposes. Pg_column_size(row(0::SMALLINT, 0::BIGINT)) AS byte16 Pg_column_size(row(0::SMALLINT)) AS byte2, We can see that with this example: SELECT pg_column_size(row()) AS empty, This means even the value of 0 requires 4 bytes of storage.įurther, Postgres is designed such that its own internal natural alignment is 8 bytes, meaning consecutive fixed-length columns of differing size must be padded with empty bytes in some cases. Thus a standard 32-bit integer which can store a value of just over 2-billion, must be read as a whole unit. In most low-level computer languages like C, in which the venerable Postgres is written, data types are addressed by their maximum size, regardless of how large they actually are. However, there’s an aspect of storage that escapes inspection almost without exception, and it’s hiding in the shadows between the columns themselves. One of the elements which requires management is storage. When working with database capacity planning, there are a lot of variables to consider, and Postgres is no different in this regard. PostgreSQL 9 Cookbook – Chinese Edition.PostgreSQL Server Programming Cookbook – 2nd Edition.PostgreSQL 9 Administration Cookbook – 3rd Edition. ![]()
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