== problem
Changing the size of a varchar field requires an ACCESS EXCLUSIVE lock, that will prevent all reads and writes to the table.
Don't Do This - Don't use varchar(n) by default
== solution
Use a text field with a CHECK CONSTRAINT makes it easier to change the max length.
Instead of:
sql CREATE TABLE "app_user" ( "id" serial NOT NULL PRIMARY KEY, "email" varchar(100) NOT NULL );
Use:
sql CREATE TABLE "app_user" ( "id" serial NOT NULL PRIMARY KEY, "email" TEXT NOT NULL, CONSTRAINT "text_size" CHECK (LENGTH("email") <= 100) );