Prática de Shadowing: Proximity Search & Geospatial Indexes Explained - Aprenda a falar inglês com vídeo
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Your database can easily find every user between age 20 and 30 in milliseconds.
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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.
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20 sits next to 21, sits next to 22, they're all on the same page.
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So everyone between 20 and 30 is one seek and a short read.
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Location doesn't work that way.
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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.
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That's why Redis can store a geohash of 52-bit integers and Postgres can store it as text.
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And both indexes behave exactly the same way.
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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.
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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.
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A nearby query is just a Z range by score against that sorted set.
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Basically, a range scan over the geohash integers.
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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.
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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.
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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.
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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,
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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.
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Every production spatial index falls into one of two types.
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The trade-offs between them are pretty clear once you know what to look for.
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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.
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These really shine when you care about shapes and accuracy.
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An R-tree doesn't just index points.
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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.
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So you can't do this in a generic setup, and writes are expensive.
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R tree inserts run a rectangular packing heuristic to minimize that overlap we talked about earlier.
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And BKD trees are basically write once since they're designed for immutable segments.
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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.
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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.
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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.
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Thank you.
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Vocabulário e dicas de fala para esta lição
Esta aula de conversação de nível C1 usa o vídeo “Proximity Search & Geospatial Indexes Explained”. As palavras que mais se repetem: tree, cell, query, database, disk. Este vídeo tem 243 frases e 3302 palavras para praticar shadowing. A fala dura 17:06. O falante fala rápido, cerca de 193 palavras por minuto, então espere sons ligados e reduzidos. Apenas 74% das palavras estão entre as 3.000 mais comuns do inglês, por isso o vocabulário é exigente.
Vocabulário principal deste vídeo
As 15 palavras mais avançadas do vídeo, com pronúncia e significado:
| Palavra | Pronúncia | Significado |
|---|---|---|
| query substantivo | /ˈkwɪɹ.i/ | pergunta |
| disk substantivo | /dɪsk/ | disco |
| integer substantivo | /ˈɪn.tɪ.d͡ʒə(ɹ)/ | inteiro |
| longitude substantivo | /ˈlɑnd͡ʒəˌtud/ | longitude |
| rectangle substantivo | /ˈrɛktæŋɡl̩/ | rectângulo, retângulo |
| hexagon substantivo | /ˈhɛk.sə.ɡɑn/ | hexágono |
| latitude substantivo | /ˈlætɪtud/ | latitude |
| spatial adjetivo | /ˈspeɪ.ʃəl/ | espacial |
| polygon substantivo | /ˈpɑliˌɡɑn/ | polígono |
| pointer substantivo | /ˈpɔɪn.təɹ/ | ponteiro |
| scan verbo | /skæn/ | varrer |
| encode verbo | /ɪnˈkoʊd/ | codificar |
| metre substantivo | /ˈmiːtəɹ/ | metro |
| proximity substantivo | /pɹɒkˈsɪm.ɪ.ti/ | proximidade |
| sphere substantivo | /sfɪɚ/ | esfera |
Phrasal verbs que você vai ouvir
| Palavra | Significado |
|---|---|
| end up verbo | terminar, acabar |
| come up with verbo | pensar (em), alcançar |
| fall over verbo | cair (sobre), tombar (sobre) |
| take over verbo | tomar posse de, apoderar-se de |
Frases que vale a pena repetir
Frases curtas e completas do vídeo que você pode usar na conversa do dia a dia:
- 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?
Gramática neste vídeo
As estruturas que o falante mais usa, com as palavras exatas do vídeo:
| Estrutura | No vídeo |
|---|---|
| Orações relativas who / which + oração — informação extra sobre uma pessoa ou coisa | structure, which means · wrapper, which is · overlap, which means |
| Present perfect have/has + particípio passado — uma ação passada que ainda importa agora | have spent · haven't loaded · we've looked |
| Voz passiva be + particípio passado — o foco está no que acontece, não em quem faz | is built · are built · been tuned |
Pronúncia para ficar de olho
O falante usa 40 contrações e formas reduzidas, como they're, you're, can't. Diga-as na forma curta, do jeito que você ouve.
- Os sons de “th”: algorithm /ˈælɡəɹɪðm̩/, thumb /ˈθʌm/, width /ˈwɪtθ/
- Os sons de “sh” e “zh”: spatial /ˈspeɪ.ʃəl/, insertion /ɪnˈsɝʃən/, dimension /daɪˈmɛn.ʃən/, limitation /lɪmɪˈteɪʃən/, collision /kəˈlɪʒn̩/
- Palavras longas — acerte a sílaba tônica: proximity /pɹɒkˈsɪm.ɪ.ti/, hierarchical /ˌhaɪəˈɹɑːkɪkəl/, predictable /pɹəˈdɪktəbləl/, limitation /lɪmɪˈteɪʃən/, dimensional /daɪˈmɛn.ʃə.nəl/
Sons difíceis para falantes de português:
- Consoante final — sem acrescentar um “i” depois: disk /dɪsk/, longitude /ˈlɑnd͡ʒəˌtud/, latitude /ˈlætɪtud/, node /noʊd/, encode /ɪnˈkoʊd/
- /l/ final — a língua toca o céu da boca, não vira “u”: spatial /ˈspeɪ.ʃəl/, hierarchical /ˌhaɪəˈɹɑːkɪkəl/, predictable /pɹəˈdɪktəbləl/, dimensional /daɪˈmɛn.ʃə.nəl/
- /h/ — um sopro suave, diferente do “r”: hexagon /ˈhɛk.sə.ɡɑn/, hierarchical /ˌhaɪəˈɹɑːkɪkəl/, heuristic /hjuˈɹɪstɪk/, workhorse /ˈwɜːkˌhɔːs/, behave /bɪˈheɪv/
Como praticar com este vídeo
- Ouça o vídeo inteiro uma vez sem falar e anote as palavras que você não conhece.
- Comece na velocidade 0,75×, faça shadowing frase por frase e volte à velocidade normal quando ficar fácil.
- Grave a sua voz e compare com o original, prestando atenção a palavras como query, disk, integer.
O que é a Técnica de Shadowing?
Shadowing é uma técnica de aprendizado de idiomas com base científica, originalmente desenvolvida para o treinamento de intérpretes profissionais. O método é simples, mas poderoso: você ouve áudio em inglês nativo e repete imediatamente em voz alta — como uma sombra seguindo o falante com 1-2 segundos de atraso. Pesquisas mostram melhora significativa na precisão da pronúncia, entonação, ritmo, sons conectados, compreensão auditiva e fluência na fala.