I answered a question a few days ago on Stack Overflow about PostgreSQL & SQL Server btree storage fundamentals. After I answered the question, I started thinking more and more about how PostgreSQL stores data on the row. Rather than be content to live in ignorance, I went digging through PostgreSQL’s source code and some hex dumps to find out what was going on inside a row.
Getting Started
To get started, we’ll create a separate test database that we’ll cunningly call test.
createdb test --username=jeremiah
psql test -f pageinspect.sql
The next step is to set up our sample data. Here we create a small sample table with four rows:
CREATE TABLE tree (key int NOT NULL, id int NOT NULL);
ALTER TABLE tree ADD CONSTRAINT pk_tree PRIMARY KEY (key, id);
INSERT INTO TREE (Key, ID) VALUES (1, 1), (1, 2), (1, 3), (1, 4);
You can copy this and run it in psql or any other tool that lets you connect to PostgreSQL and run commands. When we run the ALTER TABLE to create the primary key PostgreSQL will go ahead and create an index for us: NOTICE: ALTER TABLE / ADD PRIMARY KEY will create implicit index "pk_tree" for table "tree". SQL Server will do something similar. The implementation is different, but we’ll get to that in a bit.
Read page items
In order to view data on the page we have two options. Option 1: shutdown PostgreSQL, open our copy of the source, fire up our trusty hex editor, and then hate life for a long time. Option 2: use the pageinspect contrib module to give you some extra functionality that will help us read page data.
heap_page_items will show us the data in the table. PostgreSQL stores table data in unordered heaps, hence the name heap_page_items. The heap_page_items function takes one input: a byte array representing the page. Thankfully, we can get this from using get_raw_page. This is what the first row of output will look like (magically formatted for your display):
SELECT * FROM heap_page_items(get_raw_page('tree', 0));
Read more: Jeremiah Peschka's