tweak readme

Author: rogerwelin

README.md

@@ -5,23 +5,41 @@ A PostgreSQL extension that enforces optimal column alignment to minimize row pa
 <img src="logo.jpg" alt="pg_column_tetris logo" width="230">
 
 
-## Why It Matters
+## Why Column Order Matters
 
-PostgreSQL aligns columns to their type's natural boundary (1, 2, 4, or 8 bytes). When columns are ordered poorly, padding bytes fill the gaps. These wasted bytes don't just cost disk space — they cost **memory**. Padding is loaded as-is into `shared_buffers` and OS page cache, meaning more bytes per row → fewer rows per 8 KB page → more cache pressure → more disk I/O.
+Every heap tuple in PostgreSQL starts with a 23-byte header, followed by a null bitmap, then the actual column data. Each column must start at an address that is a multiple of its type's alignment requirement — `bigint` needs an 8-byte boundary, `integer` needs 4, `smallint` needs 2, and `boolean` just 1. When a column's natural offset doesn't land on its required boundary, PostgreSQL inserts invisible **padding bytes** to close the gap.
 
-For a table with millions of rows, reordering columns can reclaim gigabytes of memory.
+Consider this table:
+
+```
+ boolean (1 byte)  |  7 bytes padding  |  bigint (8 bytes)  |  integer (4 bytes)  |  4 bytes padding
+```
+
+That's 11 bytes of wasted padding in a single row. Reorder the columns largest-first and the padding drops to zero:
+
+```
+ bigint (8 bytes)  |  integer (4 bytes)  |  boolean (1 byte)  |  no padding
+```
+
+This padding isn't just on disk — it's loaded as-is into `shared_buffers` and the OS page cache. More bytes per row means fewer rows per 8 KB page, more cache pressure, and more disk I/O. For a table with millions of rows, fixing column order can reclaim gigabytes of memory.
 
 ## Installation
 
+Pure SQL/PL/pgSQL — no compilation needed.
+
+### Self-hosted PostgreSQL
+
 ```bash
-# From source
 make install
+```
 
-# Then in PostgreSQL
+```sql
 CREATE EXTENSION pg_column_tetris;
 ```
 
-No compilation needed — pure SQL/PL/pgSQL. Works on any PostgreSQL instance that supports event triggers (RDS, Cloud SQL, Supabase, Neon, etc.).
+### Managed services
+
+Most managed PostgreSQL providers (RDS, Cloud SQL, etc.) don't allow installing custom extensions or creating event triggers. This extension requires a self-hosted instance or a provider that supports custom extensions (e.g. [Supabase](https://supabase.com), [Neon](https://neon.tech) custom builds).
 
 ## Quick Start