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Zedstore - In-core Column Store for Postgres - Alexandra Wang, Soumyadeep Chakraborty: PGCon 2020

Alexandra Wang
Member Of Technical Staff 3 at VMware
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PGCon 2020
May 26, 2020, Online, Berkeley, USA
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About speakers

Alexandra Wang
Member Of Technical Staff 3 at VMware
Soumyadeep Chakraborty
Senior Member Of Technical Staff at VMware

I work in many areas of the database kernel, including query optimizer, query executor, storage, distributed transaction manager, performance testing, etc. I'm passionate about building high performance databases for various workloads.

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Soumyadeep (Deep) is a developer on Greenplum's database server team since the start of 2019, working at Pivotal (now a part of VMware). He has worked on various parts of Greenplum's server and more recently, Zedstore. Also, he has an active interest in Postgresql JIT and has contributed a couple of patches in the area.

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About the talk

Zedstore is a column store for PostgreSQL that targets to reduce I/Os for queries that only need access to a few columns out of a wide table, to improve performance for analytics workloads, and to enable better compressions. It is under development and it uses the table AM APIs that were introduced in PostgreSQL V12.

This talk will cover the use cases for column store in Postgres, the architecture and design decisions of Zedstore, performance and size comparisons between Zedstore and Heap tables for both OLTP and OLAP queries, changes required in the table AM APIs to accommodate table AMs other than heap, and the latest development status of Zedstore.

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Hi, my name is Sami deep and Alexandra, my colleague. And he will be talking about that store called him. The storage for pulse. Chris, we both work at VM were on green plum. so, That's a look at what we be covering today. That's from school. Does a project, its design internals with will be doing. Some demos will be looking at performance. Numbers will be talkin about both of you be such a dominant to improve performance. Will motivate open areas of work and finally we'll talk about all you can get involved with the

project goals so he won't sit through to be a column store that every post-procedure can use. It's a very commonly requested for Enterprise Creature Feature. That's important also provide another implementation of the table emepa which is important other than he being a column store. We want to fully utilize the fact that the beer going to lay out their column boys, which means we have to take into account compression. We want to be able to compress. We wanted to be efficient as well

because Efficient decompression, you want queries on subsets of columns to be blazing fast, all of this kind of points to Olaf workloads and bolts into injection. Do you want to be first there at the same time? We don't want wealthy people, homes to suffer. We kind of want to have and support the same features with heat and this is something we've seen with cream plump as well. We had happened only tables. So you couldn't do interesting to Leeds and soon customers wanted in her sentiments, sorry, update, some plates. So, this is something you keep in mind. When we do

love that store. We also wanted to have nbcc, press safety, and all the other stuff as well. So, you let Alex talk about the design. Leverages the Timothy, m a p. I d, for example, it has eight kilobytes of 6 + 5/8. Use of the common buffer manager and while some of its more interesting Lee, Nevada, So you could think of that store table as a forest of each race, we are each way represents a column and we didn't issue a note as a block on desk. So if you have a table with M columns then you would have

an attitude Chris + 1 TI-83, has it stores, the visibility information of each taco. So, all of the pieces are in the next buy a 64-bit integer, cop that store. Tid, we are going to court Ted, for most of the rest of the presentation vs, kid has a one-to-one mapping with the Adam Poynter, so we can easily leverage the winter that is passed down from the table and because of that Lying for the nose is that yesterday is Shirley logical. It means that it does not tie a topo to a physical location that has

to mister Taco in amount different different pages. When we do compression and be prepared, Create a table for us to play. Best create a table food with call anje, using the store. And that was when he started the same day that there you can. Also set the default cable access the entire fashion show you the one to one matching between CST to PST. So it just hurt because to the block number * 128. No serious, love, nice function blocks for table food. So you can see there's usually blocks for IC number 0, for the candy tree, and then

one and two formation with rate from the special space of a different pace. So now let's just look at I can do number. I just look at 1, I feel. So, this is what I have in this Darwin, you can see, block number 9 is Rock covers, all the T IDs, and then it has to switch three children, block numbers, 6/8, and 12, for each of the blocks. It has, it has the next block number and the low-key and high-key that covers the, the, the Rangers start, some low-key and high-key, and then, we can see the number of items

and we'll talk about what items are fixed. And then you can see the actual size of the block and the uncompressed size of the data, that is actually pretty good for this song called So is this Buffet pages? And it also also the future Pages? Take the majority of the is the majority of the cases for the table. But we do have other types of pages within the same room, so you're given a table on the block number and tell does the type of pay does the type. The peace pipe is blocked. So this query shows us of each

type of blocks for this table and defy them. And then run the sink where you can see that. The three of us are converted to a, a new patient. So these are actually all the times we had, For a better table. Now, let's go back to this place and I'll explain what these hearts are. So they might have page. Stores older block numbers to block numbers of older youth blocks for each attribute Rihanna. The free pages are the biggest we pre-allocated were recycled from

other types of potatoes. The Underpants store Steve visibility information of a taco and his pointed by a item on the dispatch of the TI-83, and then finally girls post pages. Alright, so in this light will talk about what an insert of a single roll looks like for that store. So we trying to insert the room, hello, 42, and all that into the table. First thing that needs to be done is the to pull buffer or responding to fool, needs to be located in the back ends. Back

in private memory by the real ID. So the Superbowl for acts as a space where we can stage a Data before they before it gets pushed onto it. Shared buffers. So the first thing that we have to do after locating the triple waffle racer, give this new trouble at 8, you can take the tape from the dead reservation, if you want, it is available there. That's what we have to do is insert a single did into the tiptree. Let's say we get five out of that. 5 is the news yesterday for the for the.

So once you're armed with that did, what we can do is we can push each datum along with that it into the corresponding at offer for the attribute involved in the answer in which case we have. Hello and 42 being pushed down to these at the first for every action Beauty you have enough of her. So far is East is staging area for all of the items for that attribute. Now, it is, I who has also two layers. First, there is a buffer of 63 parallel, arrays of kids tea time zone is Knox. In which

men try to store five and hello 5 and 42 respectively. If there is not enough space in this before. However, what we'll do is sorry, What you do is push the data into the action before the action buffer is an in-memory representation of all of the of all of the data that you can find on an attribute reading these beach, and this is what it kind of looks like. So, we pushed it up there, you can see your representatives. And this is the direction between culture and language act

with presents, the actual items and things like that. So we go into the details later. So finally, what will happen is, you're asking both of his phone, Dan with a csbg aquaflush, you'll actually flush the data on the beach. So, this is how I flush me happen. Not only by an insert Bots, from other sources as well. For other reasons, as well. We might need to flush these buffers anyway, so this is how the data ends up into that tree is on the tree. Looks like

now we'll talk about what projection looks like. So for sequential scan for a column store. It's very important that we passed on what columns need to be scanned. Otherwise we'd be ending of scanning the entire people. So here we have introduced a new table and maybe I called begin scan with Colin projection for this purpose. So next, We look at what's X Dory a.m. get next lock. Looks like so first thing. Function doesn't which is basically going to return a single drawer thing that it forced us has caused and does a scan on the tree to get the next in the scam

then it calls after fetch app for every attribute involved in the scandal in which case we have select ing. So the two attributes with the kid that was returned. So in here we return the DSM e which corresponds to the gate 5042 for this other attribute finally we have all the attributes the details we need as well and visibility information. So you feel all that up and return the start and there you have, it was returned. Okay. And look at the page layout of the talk about and then we'll

talk about the exit. So for the tid it has pretty similar structure with a regular toothpaste. It does take advantage of the special space at the end of the page to store a DSP Twitter page of text structure where we have all the Beatrice disinfect. Meditative, I'm actually already shown you what they are from the PVCs. and then between pedia pedia /, and pretty special, with store, a array of VSP idea, real So we didn't use item, we have a header that stores, the first head and lasted for Desiree item and then the

sizes of the other sex as well. So far, and then following theater, we store a sets of Yetis. So know that we don't do compression or we want to, we still want to store things as complex as possible. So we did. So we have done this simple AP. Encoding for the group of the T IDs with store where we have the first tid with the absolute value. And now it's worth without bad news for the rest of the day. And then we have under Sarah and I'm just left work. So I'm not lost stores. The

tuatha to unique and the pointers that are shared among all the T, IDs we have in this item. There are two special and appoint. Her one is always about to all the transaction and the other is that And then in the end slot worse, we actually store in Texas to this array of and was lost for each of the yetis. So I see the matching for The Logical content of a tid, a real, mm, and the physical. Next, let's look at the face. It has the same dies before they do cake by the data

for rent. The rest of the page layout, we have two streams of data stored on a case. They both RCs at stream. So the Astron between take Heather and PD lower AR lower screen, and it is uncomfortable and UCSF. Slim arms between PD, upper and puny special is caught up her ice cream and it is compressed. Soul phrase. Yes, at stream. There is a Heather where we store the size of the stream, whether it is compressor run out and the last heard of it I'm the difference

between the upper and lower stream, is that the lower stream store a array of topics? I'm following a Heather for rise for the Opera stream. We saw the same song, but they are in the compress format. Let's look at what we have in a talk so we didn't we. Again had was two, are the stats of T IDs for this song is kind of weird. Kind of use the same strategy as what we did for the PSP that Ray item where we store and their values. And the following, the tests we have the actual cause of data.

And here, you can see, these are the matching between the mountain and the actual in Cody we have Now, let's look at what we have done for the oversized Adam. So dilam is over-sized, its oversized when it cannot fit on a block. And in that case, we first pass, the volume to the total compress that infection and it's the same functions that he table uses to compare them. So we do CPR, compressions are compressed from there. If they're, that will look at the size of the datum again, the compressor again. And if the compressor fit on a block, then wait, we just ride up

to the top as a compress Adam. If it does, still does not fit on the Block. Then we write the compressed, Adam to the external to write a compressed. Adam 280 space for a side of toast pages, and then we store the pointer to the height of the tallest place in the in the car. So noticed that for both in line compression and external told Paige. We store the heat compress that them there. So when we actually compress the time we actually have double-compression wise and then again

You have no everything we like dusk. I'll hand it over to deep to explain. Why was working this way, and how we do commercial So so we'll be talkin about how your data from your Xtreme buffer would actually end up on your feet, so you've calls. Yes. Her flush. Which is called pspt asteroid, which is kind of the routine that handles all of this. So we talked about how things get into the uncompressed video of the page versus when things, get into the upper compressed and uncompressed area of the pitch. So the first thing that we need

to do is we need to descend to the Target page. Now the extra buffer comes with a range of dates so it's kind of easy to look at that first. Once you do, look at that then you check if your lure at stream, the uncompressed. Has enough space to accommodate all the potatoes if you want to answer. If you do have enough space great just defend them to the lower screen and however, what happens if you lower at she has stuff in it. And then you have to go and check your upper Street, which is a compressed area, is it. If it is, then we can't accommodate

accommodate the items from the a string buffer in the dark. So, we need to create a new page when we do fit need tune-ups. We make the conscious decision to fill out report. Everything, inside the compressed reason so be compressed that stuff from the basketball for tickets and put it in the Opera stream. This is what we call a repack operation, know with me. So happen that we can't accommodate all of these rooms in the Target page, which can happen, because an ice cream. Buffer is arbitrarily large. So, it's larger than the size of the

beach. So, it's in that case, we have to call Via sweetie after add again, and again. So, if you have to be cold in the loop, So let's back up a little bit. What happens? Now if you're up or stream is also has two fuel or extreme also has stuff anyone drinks or something of the beach. Then you what you do is you take all the details that you have in your afternoon before you take the opposite extreme uncompress it take to lure a stream and create more March, everything together and create a new ice cream buffer and then kind of follow the

same procedure. Where are you? Go down to the Target Beach and check if the Opera she was first. If it is not full. Means you can put stuff into the into that page. Now, now, when you do that, you can, you always read back the page, that means you come prep, you put everything in the compressor fusion and you do this in the loop until you've made enough progress at any point in the loop. If your target which doesn't have any more space. Y'all look cute on you. And I need to run the same thing. You did all of that big at you before and put put them

onto the starter. Put them on to the compressed area of the pitch repeat. So as long as you mean peace in a bullfrog dress, then you can continue on this know otherwise you come down to the same position box and meet the same decision as to whether you will need to call cspd. I tried again and this is kind of the flow of things of how things end up on the beach. No, let's talk about performance. So, the first thing that we want to examine is what the oldest footprint. Looks like for that store vs deep

inside reasonable skill. So, currently we have in this light stuff from TPC DS, some tables for tables with scale 50. So we see that. If you compare these two columns side-by-side, the first column in the first Deathstroke column, you can see that be kept reasonably. We got half of the sets portable desktop of this week. However, when you ramp up the pee pee, which is the number of concurrent copy sessions writing into the same speech is writing story writing into the CMT, will

you end up with you can see that the beach for? That's the size required for. Is that still goes on here. To mitigate. And the reason for that, is there some inefficient piece that's going on, because every session is trying to brighten, might be trying to write into the same attribute leave. There are some out of order, didn't start causing inefficient pizzaplex more details in the thread. That is linked to your house and we'll fix it is to reserve dates to reserve chunks of the three species of influx of an

Australian is Monte inserted reservation factor, which should really be a cock right now, but we're making a considerable anywhere. As we have increased and you see that the piece requirement goes down drastically. In fact, we even beat what we get for serial effectively eliminating, the problem by increasing and you can eliminate this problem. It's very workload sensitive. So you need to pick up carefully. Nice side. Effects of all of this is the degrees in Copier on time, which is the time taken to copies with to load the

data with 16 parents into the stables and all of these people sticking together, PC of decreased from hundred minutes. All the way to 7 minutes. This can be kind of attribute it to the decrees and contention on the tree because we are requesting blocks of kids. We don't know you're not making as many trips into the tree as before kind of effect. All right, so we have also done some performance testing of single column productive, because that's where a column

store is supposed to be. So, here's our settings, we use the TV, CVS store, sells table, we use the LG 4 compression acronym. We have still 270, which is about 102 gigabytes of raw data for the table. We run, it has two types of strokes and we have run post Arrow, copy, and concurrent talkies with multiple to train on the performance for loading. The most important got is the track. I was having that is used to show explain analyze. I was having an affair Is select a single column from store sells and we recorded the real return from d. I o x field.

And for the result, every time before we run the car, we wait restart the database and to flush the buffers and clear g o s h. So here are some results. First, let's look at SSD really, no difference between cereals and some Carl offers for SSD. And I see that store table size is a little bit further than half of the size of the heat table had the reader. We need is only one point for the device rather than 112 gigabytes for the blocks, for the particular call him, and also that she had to call him. The Isle time is about one-third of the Alice Humphrey. Luke has a

speed slower speed but because is it need to read Moc left. So the over Ohio time is still three times better than the results for HDD. So as we can see that I owe. Tom is a lot more, a lot more than the table. We read the same size of locks, the number of buffers. However, it is very slow, that indicates that we are doing a lot of running braids from the table and rotational hard disk is not good. I rather braids So why do we have? Why are we doing running break from that store. Let's go back and look at our beloved table. So so

if you look at the blocks of each other build a block and I can remember one is that for doing well, and all the blocks of the leaf plus are actually a list of logical. Number of the blocks are sequential. But if you look at the numbers 0 and a number to the Block's numbers are not consecutive to each other. If you look at the excess present, for a sequential scan for a ice build, for example, for the teach way, we do 413 7-Eleven on Friday to redo 96812, as you can see, where we have to be running wrist, and this is actually a pretty narrow table, and you

might do that. If you have a much wider is a bowl, then you would have ever built cages PetSmart interleaved with each other and What we observe is that which one would you, Chris? Wait more, you Chris the number of loading positions to the random race happens, a lot more. So in summary, HDD, suffers more from random race than me and Andy can Carlos. Actually magnifies the actual test run them. To reduce the page Randomness. Here's what we have done previously. We have one single prepaid map.

That is shared among all the trees and the cherry tree. We introduced the maps where each attribute has its own. I mean addition to that we introduce a real option for death row, table caught that Sorrel extension Factor. So whenever it is difficult because we don't want to do it for as I see, we want to do it for mostly HDD. So what it does is that whenever the Death Star relation need a, a new blog 1, a request from the storage manager. And inside of requesting, just

one block. This number of blocks, and it was a creep and pretend, the X Roblox that you passed it to the Asheville libel action. So in this way, waiting for the, we reduce the randomness for you, I would do. So whenever a I sure do need to get a new block. It will first look at the forest is not. And if there's a free haters there that I use the free. So this is how we would like some kind of results. Option to arbitrary number. That is greater than one, we have more data points.

We'll just do this for the sake of the presentation. So as we can see with, do not have, we do not see any Improvement since starting the option for RSVP for HDD Improvement. When citing The Rock's entrance to a relatively higher number, as you can see, the red Speed Racer 5 more than 500 megabytes per second. That is basically, So, so far, all the result of strong is 4 cold to run the carburetors one without any trash in the database were from the file system. What I really want to highlight is that if

we run the same car, re the second time for that store and actually take zero time for 0, Iowa time, because the size of the, the size of the blocks, I need the number of fox, and me to read is so small that with his 101.45 presets, in the sheriff's offers where we have set to 10 device. So this is where I think that's already shine. So indifferent to the single column for Jackson. We have also done the similar tests for Celexa star. We know that this is not what is Colin store is supposed to be cool. Still, we wanted to

improve the performance, they are as you can see when we start this around Staunton saxer tomorrow. Night 6, we have much Lower Rio time comparing from before I'm so everything I have. So here, if you are interested, we have a heck, Restless threat that has more data points and another Alright. So let's talk about the storage birthday. Sweet that we have four sets tore. It gives you a nice going to rerun. It doesn't have too much of data items, like 32 Max, and

someone give me, nice, holistic view of. How's that store is performing this color represents good results. And discolored person spot results. If you see your own Leisure, most of the results we are due to perform. Well, for the Olaf said, Olaf stuffer and some of the old hippie stuff we don't do that. There's some room for improvement or toast is one of this motivates. Open areas of work, special specialty relation to Performance. One thing I'd like to point out. Before I begin, is that all of these are links to the Packers trade specific messages. So you can follow

After the presentation. So let's talk about some of these index on these cans more efficient right now. They just are index cans so you can prove that we need to be proved right now for brain index and index of a point to arrange that Matt McCann map to multiple has extra fries. Instead of water expected. Requirement one-page it can map to multiple. So that's something that we saw on the previous slide really quick. Just do two pizzas for updates and deletes as well as there can

be like the final toast. Be sure those pages can be extremely empty and underutilized, which causes the blood we need to fix that. Going forward, we want to avoid full keyboard rewrites for altar table operations. Example, for drop column. There's no TV Olympia right now. Invoked when you drop, we would like, to be able to reclaim, you want an API, so that we can look into that and reclaim all of the Beatles for a particular, I should be free. Whenever you drop the column, we should just reclaim a. Similarly, other operations

touching a few of the call of a few of the columns, are a single column, should not affect the entire table, and two leads for two things updates. If you want to update two columns of a few hundred kilometers wide table, right now, you have to talk to all the 200 attribute trees because you have to go in and update all the kits for for the Beatles in word, for the rules involved in the update, which is bad. Seminole, leader Steven to bullfish Rovers on anything that caused this thing's like a check to eat or be able to lock and things like that. They expect for raw. Now,

we don't need that special, especially if you don't need that, you don't need that. Because we all only need. All we need is a pet scan. So this is some EDM Epi Improvement as well as increase right now. Midway inside an accident, according process, because the because we are sometimes looking for a single parent, that doesn't mean that we have to decode the entire a street, you should probably have to do. So more of the future work mountains, official work. If you want to be able

to support column families, read architecture, Scott supports that completely. All you have to do is go less tedious, Tetons, belonging to various other columns into a single single attribute tree. At the other end of that spectrum is a compressed Rule Store in which you have a big condom family, comprising of all of the columns that that's kind of neat and you can get out of the box. All of the architecture will be the same as we have to still be expected and song. Then you won't to be able to reuse.

It's being run out of tits because 40bits, you want to be able to recycle order, death deaths caused by the recent updates, in the long term. We want to replace our undoing permutation without scheming to intimidation right now. Or I'm on underemployment Visibility information, you want to be able to make that change, making the metal piece overflow right now. The number of columns, if you can support is limited by the amount of space in the metal Beach because of Metro PC doesn't know what floor, we have only one message finally going to recognize

free, space inside or 34 pages right now, it's too big to Norm granular at the beach levels of everything, everything inside a piece on unusual eyes then, and only then is it, a free page compression. A lot of discussion on the Upstream thread regarding compression, different strategies for compression, including dictionary best compression and do something that hottie from Microsoft suggested as well. So we want to be able to experiment with all of that. Finally, we have to make planner aware that there's a Store. Some of the work we have done, we can go on to the next slide, but mostly things

like physical tea. Last optimization doesn't work with that store. So these are some of the related hackers trades around the main sets to threat. That we've contributed to the first to relate to passing down the list of columns, getting it from planner, and they're passing them down to the scam because we don't want to scan everything, but we have a projection best. Similarly, we want better estimate of the number of pages for queries, that only require a subset of columns and this is something that's in this tribe and then using the statistics and planner to get better plans

to build. You can use to play with that story of the GitHub repo inspect functions. We've doubled. There's an ansible playbook for skin testing. There's a storage worth the slide, a bit to all the results. You can get that. You can run to request us with these two extra options. You can get involved here, you can join the meeting with red arms that store, you can sign up on the set store challenge in Plum. Slack and finally thank you to everybody who was being involved in the project was

contributed in the thread and then going to kill you and I feel free to drop your questions also feel free later on if you want to drop more questions and thank you so much. Bye. A live with Alex and some it and we're going to give the answers to the questions were raised over to you. Okay, so we see one question, what was that store become an extension? Or does it need to be part of 4:00? So, we want to have a columns as in core of post, but it doesn't have to be that store. We are targeting. We would try to have all the functionality. We're targeting to be

able, able to be part of the crew. We don't know whether it's going to become extension for California. At the end of the day, we want, you want something that any post receives, it would be able to use, it would be nice to have it in the car coming in and doing the cute Q & A session. I know that doing the talk offline and recording it at home is not easy, not easy at all. Thank you for doing that. You for organizing. You're welcome.

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