ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: Proximity Search & Geospatial Indexes Explained
กำลังสร้างบทเรียน...
1
Your database can easily find every user between age 20 and 30 in milliseconds.
2
But when you ask it to find every driver within two kilometers of a rider for an Uber-like app, it falls over.
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Now on the surface, these two queries look really similar, but they're actually very different.
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The age query is fast because a B tree keeps sorted keys packed together on disk.
5
20 sits next to 21, sits next to 22, they're all on the same page.
6
So everyone between 20 and 30 is one seek and a short read.
7
Location doesn't work that way.
8
You have two numbers, latitude and longitude.
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And what you care about is the straight line distance between two points.
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Two B trees don't save you here either.
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One on lat gives you a horizontal strip of the earth.
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One on long gives you a vertical strip of the earth.
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And their intersection is still millions of rows
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that you have to check the distance between them and your point of interest by hand.
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Now, the real problem is that 1D sort order doesn't preserve 2E closeness.
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Take two cabs parked five blocks away in midtown Manhattan.
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They're neighbors.
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But if your 1D index sorts on longitude alone, every other cab in the city at a longitude between them, from Bronx down to Staten Island,
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gets crammed into the index between these two neighbors.
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They're a block apart on the street and millions of rows apart in your database.
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Any naive flattening of 2D into 1D rips apart the relationship we actually care about.
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Now every modern production system, Postgres, Elasticsearch, Redis, Google Maps, Uber, they've all converged on one of two approaches to solving this proximity search problem.
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Either build a custom tree like a B-tree but specially tuned for location, or turn latitude and magnitude into a single key that a plain B-tree can already sort.
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Why is B-tree the prize?
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Because it's the one data structure databases have spent 50 years tuning.
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It stays balanced as data changes, it packs sorted keys into disk pages so a range query is one seek in a sweep, and every database you'd ever reach for our E-ships one.
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If you can squeeze your spatial problem into a B-tree shape, you inherit all of that for free.
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Now, this sounds more complicated than it is.
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Both approaches end up at the same price, a balanced tree of pages on disk that you can range scan.
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Let's walk through how we get there.
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The first serious attempt came in 1974 from Finkel and Bentley.
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They called it the quad tree, and it's an incredibly useful foundation to understanding proximity search.
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Take your whole map, split it into four quadrants.
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If any quadrant has too many points in it, Split that one into four again.
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Now you can keep doing this recursively until every leaf has a manageable number of points.
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To find points near a location, you just descend that tree one level at a time.
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At each node, you compare the query points latitude and longitude against the cell's midpoint.
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North or south, east or west.
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That picks one of the four children and you recurse into it.
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You then repeat this until you land on a leaf.
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Then you check the neighboring leaves too to catch anything that happened to sit just on the boundary from your query point.
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What's really nice about this is that it adapts to density.
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Manhattan ends up as a deep tree because it's full of pins.
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A lake upstate stays a single core cell because nothing there.
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We don't waste a bunch of space indexing the ocean.
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But this has a really bad downside on latency at scale.
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Because these splits always happen at the geometric midpoint of a cell, not where the data actually sits,
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a dense cluster like Manhattan just keeps getting sliced in half over and over and over again until those points finally separate.
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You burn tens or more levels of depth in your tree before you actually reach a leaf node, whereas a query in Vermont might have a depth of just two.
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That means that your query latency depends on where in the world you're looking.
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You can't reason about worst-case performance very well, and the hot regions, the ones that you actually care about, like Manhattan, are the slowest.
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There's also a disk problem.
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A quad tree is a pointer structure, which means every node in the tree lives at some arbitrary address in memory, with pointers linking to its four children.
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That works beautifully when the whole tree fits in RAM because following pointers is a really cheap memory lookup.
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But databases can't make the assumption that the whole tree fits in memory.
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They read data from disk and fix page sizes
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and every pointer you follow has a good chance of pointing you at a page
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that you haven't loaded yet which forces random disk reads every single time.
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A few hops in and you're spending most of your query time waiting on the disk instead of doing useful work.
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Now you still see quadtrees today.
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Google Maps uses them for map tile systems, game engines use them for collision detection, But for a scale database where the index has to live on disk and stay efficient when it doesn't fit in memory,
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we need something smart.
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Well, one year later in 1975, the same guy, he came back with a binary cousin he called the KD tree.
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The idea is that instead of splitting all dimensions at once into four quadrants, alternate.
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At level zero, split on X.
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At level one, split on Y.
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Then back to X, back to why you get the picture.
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One dimension per level cycling through.
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This gives you a binary tree for multi-dimensional data that's balanced, importantly, if you split at the median.
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And it was the workhorse for nearest neighbor type searches for decades.
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But KD trees, they have the same disk problem as quad trees.
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There's pointers everywhere.
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There's no clean mapping to pages.
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And so great in memory, but painful off of it.
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The modern fix is what's called a BKD tree, short for block KD tree.
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Instead of one point per node, points get packed into blocks sized to a disk page.
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And the whole tree is built once from a batch of data.
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Elasticsearch actually uses this for their geo fields today.
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So when you run a geo query in Elasticsearch, you're hitting a 50 year old algorithm with a disk friendly wrapper, which is pretty cool.
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Next, in 1984, Antonin Gutman at Berkeley, he came up with the first spatial index designed specifically for a database.
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Guttman had two problems that Quadtrees and KDtrees hadn't handled.
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The first was shapes.
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Every index before this one assumed your data was a point, like a driver or a pin or a user location.
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But real geographic data has lines and polygons.
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A highway is a line.
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A country is a polygon.
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A Starbucks might be a point, but the Starbucks delivery zone is a polygon.
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And so you can't meaningfully drop a country into a Quadtrees cell.
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The second was the disk issue.
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Guttman wanted the spatial analog of a B-tree.
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Balanced, fast to read off of disk, fast to update when drivers move, the pointer-heavy in-memory structures weren't gonna cut it anymore.
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His answer was to use minimum bounding rectangles.
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Every single object gets wrapped in the smallest rectangle that contains it, with sides parallel to the latitude and longitude axes on the map.
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A point is a rectangle with zero width and height.
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A highway gets a long, thin rectangle around it, and a country, of course, gets a big rectangle that bounds it.
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Then nearby rectangles can get grouped into larger enclosing rectangles.
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Those group into bigger ones all the way up to the root.
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The clever part of this is that this tree stays balanced like a bee tree.
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Every leaf lives at the same depth so every query touches the same predictable number of pages from the database.
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Every node fits into a disk page, inserts and deletions rebalance by splitting and merging pages just like a regular bee tree does.
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There's one trade-off to call out here.
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Unlike quadtree cells, archery rectangles can overlap.
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So if your query point lands inside two bounding rectangles at once, you have to descend both branches and check both subtrees.
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So a bad insertion strategy produces lots of overlap, which means a lot of branches to follow, which tanks performance.
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Archeries are the workhorse of production spatial indexes.
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PostGIS, SQLite, Oracle Spatial, they all use an archery variant under the hood.
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Six years after Gutmann's paper, a team in Germany published the R-Star tree, which uses a smarter insertion algorithm to minimize those overlaps we were just talking about.
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And that insertion heuristic is what almost all of the modern R-Tree implementations are built on today.
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Now, back to the video.
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Okay, still with me?
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Let's take a quick second to zoom back out.
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Everything we've looked at so far, quad trees, KD trees, R trees, they're all custom tree structures.
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They work, but they're complicated.
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Your database needs special indexing code, special query code, special tooling.
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What if instead we skipped all of that?
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What if we could take latitude and longitude and turn them into a single number, just one integer, where numerically close integers meant geographically close locations?
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If we had an integer, then we could use a regular B-tree, that boring thing our database already has, and where range scans just work out of the box.
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This is that second camp, and it's the idea that took over most of the industry.
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In 2008, a developer named Gustavo Niemeyer took decades-old math
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and turned it into something you can actually compute in a few lines of code.
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He called it a geohash.
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The idea is dead simple.
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Divide the world into a grid of 32 cells and label each one with a character.
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Your location falls into one of them, and that's your first character.
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Now, take that cell and divide it into 32 smaller cells.
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Your location again falls into one of those.
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That's your second character.
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Keep recursing as deep as you want.
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Each additional character zooms in further to the map.
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A five character geohash, for example, is about five kilometers across.
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Nine characters gets you all the way down to five meters.
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The final string, something like dr5ru, is your 2D location encoded as a 1D key.
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The payoff is that strings sharing a prefix are usually near each other.
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And here's the part that's really worth pausing on.
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Each character picks one of 32 options, which is exactly five bits.
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So a geohash isn't really a string, it's a stack of bits.
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The string dr5ru is 25 of those bits grouped into five letters for human readability.
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Read the same 25 bits straight through as a number, and you've got an integer.
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Same bits, same sort order.
143
That's why Redis can store a geohash of 52-bit integers and Postgres can store it as text.
144
And both indexes behave exactly the same way.
145
The huge payoff here is that keys sharing a prefix are usually near each other, which means you can query proximity on any sorted index in a database.
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Where geohash like dr5ru wildcard, that's just a regular B-tree prefix scan.
147
It's totally efficient.
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This is how Redis Geo commands work in production.
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When you call a geoadd, Redis computes a 52-bit GeoHash integer and stores it in a sorted set, which is Redis' name for a B-tree-like structure that keeps entries ordered by a numerical score.
150
A nearby query is just a Z range by score against that sorted set.
151
Basically, a range scan over the geohash integers.
152
This is one of the most elegant uses of an existing data structure that you'll find in production.
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There is one catch worth noting, though.
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Geohashes have edge cases at cell boundaries.
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Two points a meter apart can actually end up landing in completely different cells, and therefore get completely different prefixes if they straddle the boundary.
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Picture a writer standing right on the edge of a cell, dr5ru.
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The closest driver might be 10 meters away in cell DR5RG.
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A naive prefix scan over DR5RU would miss them entirely.
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The standard fix is what's called the 3x3 trick.
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You compute the cell your query point lands in, then walk to the eight neighboring cells and query all nine as a unit.
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That way you can't miss anyone within one cell's distance of you, no matter how close to a boundary you are.
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You can then post-filter the results by exact distance to drop the corners that are technically in your query window, but actually too far away.
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And almost every encoded key index in production does some version of this trick at query time.
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All right, now geohash works great when we're mapping out a city, but it starts to hurt when you look at a globe.
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The problem is that geohash treats latitude and longitude as if they lived on a flat rectangle.
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And the earth isn't flat.
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A degree of longitude is about 111 kilometers at the equator and nearly zero at the pulse.
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So a cell that's one geohash character wide is a fat square at the equator
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and a tiny sliver near each of the poles.
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You want cells of roughly equal area everywhere on Earth.
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And you want a 1D ordering that preserves locality on a sphere, not just a rectangle.
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So around 2011, Google built S2 to fix this.
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The trick is to wrap the sphere in a cube and project the Earth onto its six flat surfaces.
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On each face, you can subdivide cleanly into cells that stay roughly the same size anywhere on the globe.
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Every location ends up with a 64-bit integer called an S2 cell ID.
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And like the geohash, the IDs are hierarchical.
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Truncate the ID and you get a coarser parent cell.
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S2 is what powers MongoDB's 2D sphere index.
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When your geoquery correctly handles a polygon that crosses the anti-meridian, that's S2 under the hood.
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Flat Earth indexes treat such a polygon as stretching all the way around the globe.
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But S2 knows better.
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Okay, last one now.
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In 2008, Uber open sourced H3, which they use for dispatch and surge pricing.
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The twist with H3 is that it uses hexagons instead of squares.
185
Why?
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Well, a square has two kinds of neighbors, four sharing an edge, but four sharing only a corner, and the corner ones are further away.
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That asymmetry makes heat maps and everyone within NSTELS of me queries messy.
188
So a hexagon has exactly six neighbors, all at the exact same distance from each other.
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And so it's clean math for the kind of analytics that Uber runs every single day.
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H3 tiles the entire globe in hexagons, and each hexagon roughly subdivides into seven smaller hexagons.
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So you can zoom in as far as you need, just like with S2 and Geohashes.
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You get 16 resolutions in total, from continent-sized cells down to about a square meter.
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Every cell gets a 64-bit integer ID.
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And again, like Geohash and S2, the IDs are hierarchical.
195
Zero out the end of the ID and get a parent cell.
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Now you might be wondering, how do you sort a hexagon in the first place?
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And the honest answer is that you don't sort the hexagons themselves.
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You sort their IDs.
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So H3 walks the hexagon grid in a deterministic order, kind of like a space-filling curve,
200
and it assigns each cell along that walk an integer that gets larger as you move along the path.
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Math aside, the result is the same property GeoHash and S2 give you.
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Numerically closed IDs are usually geographically close to each other, so that a B-tree can scan over those IDs, giving you a clump of nearby hexagons.
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Now Now, the way that it works in production is that every driver reports their location, and the system converts that latitude and longitude into an H3 cell, say a 200 meter cell.
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When a rider opens up their app, you get their latitude and longitude, find which cell they're in, expand to the six neighbors around them, look for any drivers.
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If you need a wider net, expand to the cells around those, and continue this until you find the closest driver.
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Okay, let's start to summarize what we learned.
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We threw a lot at you there.
208
Every production spatial index falls into one of two types.
209
The trade-offs between them are pretty clear once you know what to look for.
210
The first type is those custom spatial trees.
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The database ships a purpose-built tree tuned to behave like a B-tree on disk, where it's balanced, page size nodes,
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predictable depths, but it doesn't explicitly use a B-tree.
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PostGist uses an R-tree variant.
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Elasticsearch uses that BKD tree.
215
These really shine when you care about shapes and accuracy.
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An R-tree doesn't just index points.
217
It indexes lines, polygons, and complex geometries, which means that you can ask things like, does this highway intersect this country?
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Or which delivery zone contains this address?
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And you get an exact answer.
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The downside is that your database has to ship a real spatial extension.
221
So you can't do this in a generic setup, and writes are expensive.
222
R tree inserts run a rectangular packing heuristic to minimize that overlap we talked about earlier.
223
And BKD trees are basically write once since they're designed for immutable segments.
224
That makes them a bad fit for data that changes a lot, like with Uber, where you have many moving drivers.
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The second type is the encoded key.
226
You basically turn latitude and longitude into a single sortable integer and drop it into a regular run-of-the-mill B-tree, and then you're done.
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Redis uses GeoHash, MongoDB 2Sphere uses S2, and Uber uses H3.
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This approach wins out on simplicity and speed.
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It works in any database with a B-tree or a sorted structure.
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There's no spatial extension required.
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Writes are dirt cheap
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because a driver moving around is just a single integer update on an index that the database has already been tuned for.
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And that's exactly why Uber can handle millions of live location pings per second.
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The cell IDs are also hierarchical, so you can zoom out by just truncating the ID.
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The trade-off is that you're mostly stuffed with points, and you get boundary artifacts where two locations a meter apart can land on very different keys,
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which means you usually have to query a ring of neighbor cells and post filter by exact location.
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Now the rule of thumb is pretty simple.
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If you need shapes and exact geometry, reach for a custom tree, but understand the limitations on write throughput.
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If you have points at scale with heavy writes, reach for those encoded keys like geohashes, S2, or H3.
240
Thanks everybody for watching.
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I really hope you enjoyed this video today.
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If you want to learn more about proximity search, database indexing, or anything system design, head over to hellointerview.com.
243
Thank you.
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คำศัพท์และข้อสังเกตด้านการพูดสำหรับบทเรียนนี้
บทเรียนฝึกพูดระดับ C1 นี้ใช้วิดีโอ “Proximity Search & Geospatial Indexes Explained” เป็นสื่อ คำที่ถูกพูดซ้ำบ่อยที่สุด: tree, cell, query, database, disk วิดีโอนี้มี 243 ประโยค และ 3302 คำ สำหรับฝึกพูดตาม ช่วงที่มีเสียงพูดยาว 17:06 ผู้พูดพูดเร็ว ประมาณ 193 คำต่อนาที จึงมีการเชื่อมเสียงและลดเสียงค่อนข้างมาก มีเพียง 74% ของคำที่อยู่ใน 3,000 คำที่ใช้บ่อยที่สุดในภาษาอังกฤษ คำศัพท์จึงค่อนข้างยาก
คำศัพท์สำคัญในวิดีโอนี้
คำที่ยากที่สุด 15 คำในวิดีโอ พร้อมคำอ่านและความหมาย:
| คำศัพท์ | คำอ่าน | ความหมาย |
|---|---|---|
| query คำนาม | /ˈkwɪɹ.i/ | สอบถาม |
| disk คำนาม | /dɪsk/ | ดิสก์, จักร |
| rectangle คำนาม | /ˈrɛktæŋɡl̩/ | สี่เหลี่ยมผืนผ้า |
| hexagon คำนาม | /ˈhɛk.sə.ɡɑn/ | หกเหลี่ยม |
| polygon คำนาม | /ˈpɑliˌɡɑn/ | รูปหลายเหลี่ยม |
| pointer คำนาม | /ˈpɔɪn.təɹ/ | ไม้ชี้ |
| prefix คำนาม | /ˈpɹiːfɪks/ | อุปสรรค |
| metre คำนาม | /ˈmiːtəɹ/ | เมตร |
| sphere คำนาม | /sfɪɚ/ | ทรงกลม |
| archery คำนาม | /ˈɑɹt͡ʃəɹiː/ | การยิงธนู |
| equator คำนาม | /ɪˈkweɪ.tɚ/ | เส้นศูนย์สูตร |
| quadrant คำนาม | /ˈkwɑ.dɹənt/ | จตุภาค |
| neighbour คำนาม | /ˈneɪbɚ/ | เพื่อนบ้าน |
| naive คำคุณศัพท์ | /naɪˈiv/ | ไร้เดียงสา, นาอีฟ |
| variant คำนาม | /ˈvæɹi.ənt/ | การเปลี่ยนแปลง |
ประโยคที่ควรฝึกพูดซ้ำ
ประโยคสั้น ๆ ที่สมบูรณ์จากวิดีโอ ซึ่งนำไปใช้ในบทสนทนาประจำวันได้:
- Let's walk through how we get there.
- Let's take a quick second to zoom back out.
- What if instead we skipped all of that?
ไวยากรณ์ในวิดีโอนี้
โครงสร้างที่ผู้พูดใช้บ่อยที่สุด พร้อมคำพูดจริงจากวิดีโอ:
| โครงสร้าง | ในวิดีโอ |
|---|---|
| Relative clauses who / which + อนุประโยค — ข้อมูลเพิ่มเติมเกี่ยวกับคนหรือสิ่งของ | structure, which means · wrapper, which is · overlap, which means |
| Present perfect have/has + กริยาช่อง 3 — เหตุการณ์ในอดีตที่ยังเกี่ยวข้องกับปัจจุบัน | have spent · haven't loaded · we've looked |
| Passive voice be + กริยาช่อง 3 — เน้นสิ่งที่เกิดขึ้น ไม่ใช่ผู้กระทำ | is built · are built · been tuned |
การออกเสียงที่ควรระวัง
ผู้พูดใช้รูปย่อและรูปลดเสียง 40 ครั้ง เช่น they're, you're, can't ให้พูดแบบสั้นตามที่ได้ยิน
- เสียง “th”: algorithm /ˈælɡəɹɪðm̩/, thumb /ˈθʌm/, width /ˈwɪtθ/
- เสียง “sh” และ “zh”: spatial /ˈspeɪ.ʃəl/, insertion /ɪnˈsɝʃən/, dimension /daɪˈmɛn.ʃən/, limitation /lɪmɪˈteɪʃən/, collision /kəˈlɪʒn̩/
- คำยาว — ลงเสียงหนักให้ถูกพยางค์: proximity /pɹɒkˈsɪm.ɪ.ti/, hierarchical /ˌhaɪəˈɹɑːkɪkəl/, predictable /pɹəˈdɪktəbləl/, limitation /lɪmɪˈteɪʃən/, dimensional /daɪˈmɛn.ʃə.nəl/
เสียงที่คนไทยมักออกเสียงยาก:
- เสียงท้ายคำ — ออกเสียงให้ครบ ไม่เปลี่ยนเป็นตัวสะกดแบบไทย: spatial /ˈspeɪ.ʃəl/, prefix /ˈpɹiːfɪks/, hierarchical /ˌhaɪəˈɹɑːkɪkəl/, workhorse /ˈwɜːkˌhɔːs/, naive /naɪˈiv/
- /v/ — ไม่ใช่ /w/ ให้ฟันบนแตะริมฝีปากล่าง: overlap /ˌəʊvəˈlæp/, naive /naɪˈiv/, variant /ˈvæɹi.ənt/, behave /bɪˈheɪv/, interactive /ˌɪn.tɚˈæk.tɪv/
- /z/ — เสียงก้อง ไม่ใช่ /s/: zoom /zuːm/, minimize /ˈmɪn.ɪˌmaɪz/
วิธีฝึกกับวิดีโอนี้
- ฟังวิดีโอให้จบหนึ่งรอบโดยยังไม่ต้องพูด แล้วจดคำที่ยังไม่รู้จัก
- เริ่มที่ความเร็ว 0.75× พูดตามทีละประโยค แล้วกลับไปใช้ความเร็วปกติเมื่อเริ่มคล่อง
- อัดเสียงตัวเองแล้วเทียบกับต้นฉบับ โดยสังเกตคำอย่าง query, disk, rectangle เป็นพิเศษ
เทคนิค Shadowing คืออะไร?
Shadowing เป็นเทคนิคการเรียนรู้ภาษาที่ได้รับการรับรองทางวิทยาศาสตร์ พัฒนาขึ้นสำหรับการฝึกนักแปลมืออาชีพ วิธีการนี้เรียบง่ายแต่ทรงพลัง: คุณฟังเสียงภาษาอังกฤษจากเจ้าของภาษาและพูดตามทันที — เหมือนเงาที่ตามผู้พูดด้วยช่วงเวลาห่าง 1-2 วินาที การวิจัยแสดงว่าเทคนิคนี้ปรับปรุงความแม่นยำในการออกเสียง ทำนองเสียง จังหวะ การเชื่อมเสียง การฟังเข้าใจ และความคล่องแคล่วในการพูดได้อย่างมีนัยสำคัญ