A patio here sits on sand that drains in minutes and has its outside edge watered all summer, and it is the two together, not either alone, that take the edge out from under it.
The flats between Goldenwest Street and the Santa Ana River are dune and beach sand under a foot or two of topsoil, which is why the tract builders of the 1960s could pour a 3-inch pad off every sliding door and see nothing wrong with it for years. Sand holds a load well while it is held in place. It does not swell or shrink, and it drains before a puddle forms, so the middle of those pads is often still sound. But sand has no stick, and water past an open edge takes it grain by grain.
The other three edges belong to something else. The house is a slab on the same sand with its floor a hand above the yard, the neighbor's block wall stands on a footing along the line, and the old pad is usually still there, so a patio poured tight to any of them bears on a thing that does not move while the sand beside it does. Each gets its own detail, a joint, a gap or a break-out. With a pool, the pool deck is poured with it.