--- slug: /en/getting-started/example-datasets/stackoverflow sidebar_label: Stack Overflow sidebar_position: 1 description: Analyzing Stack Overflow data with ClickHouse --- # Analyzing Stack Overflow data with ClickHouse This dataset contains every `Post`, `User`, `Vote`, `Comment`, `Badge, `PostHistory`, and `PostLink` that has occurred on Stack Overflow. Users can either download pre-prepared Parquet versions of the data, containing every post up to April 2024, or download the latest data in XML format and load this. Stack Overflow provide updates to this data periodically - historically every 3 months. The following diagram shows the schema for the available tables assuming Parquet format. ![Stack Overflow schema](./images/stackoverflow.png) A description of the schema of this data can be found [here](https://meta.stackexchange.com/questions/2677/database-schema-documentation-for-the-public-data-dump-and-sede). ## Pre-prepared data We provide a copy of this data in Parquet format, up to date as of April 2024. While small for ClickHouse with respect to the number of rows (60 million posts), this dataset contains significant volumes of text and large String columns. ```sql CREATE DATABASE stackoverflow ``` The following timings are for a 96 GiB, 24 vCPU ClickHouse Cloud cluster located in `eu-west-2`. The dataset is located in `eu-west-3`. ### Posts ```sql CREATE TABLE stackoverflow.posts ( `Id` Int32 CODEC(Delta(4), ZSTD(1)), `PostTypeId` Enum8('Question' = 1, 'Answer' = 2, 'Wiki' = 3, 'TagWikiExcerpt' = 4, 'TagWiki' = 5, 'ModeratorNomination' = 6, 'WikiPlaceholder' = 7, 'PrivilegeWiki' = 8), `AcceptedAnswerId` UInt32, `CreationDate` DateTime64(3, 'UTC'), `Score` Int32, `ViewCount` UInt32 CODEC(Delta(4), ZSTD(1)), `Body` String, `OwnerUserId` Int32, `OwnerDisplayName` String, `LastEditorUserId` Int32, `LastEditorDisplayName` String, `LastEditDate` DateTime64(3, 'UTC') CODEC(Delta(8), ZSTD(1)), `LastActivityDate` DateTime64(3, 'UTC'), `Title` String, `Tags` String, `AnswerCount` UInt16 CODEC(Delta(2), ZSTD(1)), `CommentCount` UInt8, `FavoriteCount` UInt8, `ContentLicense` LowCardinality(String), `ParentId` String, `CommunityOwnedDate` DateTime64(3, 'UTC'), `ClosedDate` DateTime64(3, 'UTC') ) ENGINE = MergeTree PARTITION BY toYear(CreationDate) ORDER BY (PostTypeId, toDate(CreationDate), CreationDate) INSERT INTO stackoverflow.posts SELECT * FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/posts/*.parquet') 0 rows in set. Elapsed: 265.466 sec. Processed 59.82 million rows, 38.07 GB (225.34 thousand rows/s., 143.42 MB/s.) ``` Posts are also available by year e.g. [https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/posts/2020.parquet](https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/posts/2020.parquet) ### Votes ```sql CREATE TABLE stackoverflow.votes ( `Id` UInt32, `PostId` Int32, `VoteTypeId` UInt8, `CreationDate` DateTime64(3, 'UTC'), `UserId` Int32, `BountyAmount` UInt8 ) ENGINE = MergeTree ORDER BY (VoteTypeId, CreationDate, PostId, UserId) INSERT INTO stackoverflow.votes SELECT * FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/votes/*.parquet') 0 rows in set. Elapsed: 21.605 sec. Processed 238.98 million rows, 2.13 GB (11.06 million rows/s., 98.46 MB/s.) ``` Votes are also available by year e.g. [https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/posts/2020.parquet](https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/votes/2020.parquet) ### Comments ```sql CREATE TABLE stackoverflow.comments ( `Id` UInt32, `PostId` UInt32, `Score` UInt16, `Text` String, `CreationDate` DateTime64(3, 'UTC'), `UserId` Int32, `UserDisplayName` LowCardinality(String) ) ENGINE = MergeTree ORDER BY CreationDate INSERT INTO stackoverflow.comments SELECT * FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/comments/*.parquet') 0 rows in set. Elapsed: 56.593 sec. Processed 90.38 million rows, 11.14 GB (1.60 million rows/s., 196.78 MB/s.) ``` Comments are also available by year e.g. [https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/posts/2020.parquet](https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/comments/2020.parquet) ### Users ```sql CREATE TABLE stackoverflow.users ( `Id` Int32, `Reputation` LowCardinality(String), `CreationDate` DateTime64(3, 'UTC') CODEC(Delta(8), ZSTD(1)), `DisplayName` String, `LastAccessDate` DateTime64(3, 'UTC'), `AboutMe` String, `Views` UInt32, `UpVotes` UInt32, `DownVotes` UInt32, `WebsiteUrl` String, `Location` LowCardinality(String), `AccountId` Int32 ) ENGINE = MergeTree ORDER BY (Id, CreationDate) INSERT INTO stackoverflow.users SELECT * FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/users.parquet') 0 rows in set. Elapsed: 10.988 sec. Processed 22.48 million rows, 1.36 GB (2.05 million rows/s., 124.10 MB/s.) ``` ### Badges ```sql CREATE TABLE stackoverflow.badges ( `Id` UInt32, `UserId` Int32, `Name` LowCardinality(String), `Date` DateTime64(3, 'UTC'), `Class` Enum8('Gold' = 1, 'Silver' = 2, 'Bronze' = 3), `TagBased` Bool ) ENGINE = MergeTree ORDER BY UserId INSERT INTO stackoverflow.badges SELECT * FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/badges.parquet') 0 rows in set. Elapsed: 6.635 sec. Processed 51.29 million rows, 797.05 MB (7.73 million rows/s., 120.13 MB/s.) ``` ### `PostLinks` ```sql CREATE TABLE stackoverflow.postlinks ( `Id` UInt64, `CreationDate` DateTime64(3, 'UTC'), `PostId` Int32, `RelatedPostId` Int32, `LinkTypeId` Enum8('Linked' = 1, 'Duplicate' = 3) ) ENGINE = MergeTree ORDER BY (PostId, RelatedPostId) INSERT INTO stackoverflow.postlinks SELECT * FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/postlinks.parquet') 0 rows in set. Elapsed: 1.534 sec. Processed 6.55 million rows, 129.70 MB (4.27 million rows/s., 84.57 MB/s.) ``` ### `PostHistory` ```sql CREATE TABLE stackoverflow.posthistory ( `Id` UInt64, `PostHistoryTypeId` UInt8, `PostId` Int32, `RevisionGUID` String, `CreationDate` DateTime64(3, 'UTC'), `UserId` Int32, `Text` String, `ContentLicense` LowCardinality(String), `Comment` String, `UserDisplayName` String ) ENGINE = MergeTree ORDER BY (CreationDate, PostId) INSERT INTO stackoverflow.posthistory SELECT * FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/stackoverflow/parquet/posthistory/*.parquet') 0 rows in set. Elapsed: 422.795 sec. Processed 160.79 million rows, 67.08 GB (380.30 thousand rows/s., 158.67 MB/s.) ``` ## Original dataset The original dataset is available in compressed (7zip) XML format at [https://archive.org/download/stackexchange](https://archive.org/download/stackexchange) - files with prefix `stackoverflow.com*`. ### Download ```bash wget https://archive.org/download/stackexchange/stackoverflow.com-Badges.7z wget https://archive.org/download/stackexchange/stackoverflow.com-Comments.7z wget https://archive.org/download/stackexchange/stackoverflow.com-PostHistory.7z wget https://archive.org/download/stackexchange/stackoverflow.com-PostLinks.7z wget https://archive.org/download/stackexchange/stackoverflow.com-Posts.7z wget https://archive.org/download/stackexchange/stackoverflow.com-Users.7z wget https://archive.org/download/stackexchange/stackoverflow.com-Votes.7z ``` These files are up to 35GB and can take around 30 mins to download depending on internet connection - the download server throttles at around 20MB/sec. ### Convert to JSON At the time of writing, ClickHouse does not have native support for XML as an input format. To load the data into ClickHouse we first convert to NDJSON. To convert XML to JSON we recommend the [`xq`](https://github.com/kislyuk/yq) linux tool, a simple `jq` wrapper for XML documents. Install xq and jq: ```bash sudo apt install jq pip install yq ``` The following steps apply to any of the above files. We use the `stackoverflow.com-Posts.7z` file as an example. Modify as required. Extract the file using [p7zip](https://p7zip.sourceforge.net/). This will produce a single xml file - in this case `Posts.xml`. > Files are compressed approximately 4.5x. At 22GB compressed, the posts file requires around 97G uncompressed. ```bash p7zip -d stackoverflow.com-Posts.7z ``` The following splits the xml file into files, each containing 10000 rows. ```bash mkdir posts cd posts # the following splits the input xml file into sub files of 10000 rows tail +3 ../Posts.xml | head -n -1 | split -l 10000 --filter='{ printf "\n"; cat - ; printf "\n"; } > $FILE' - ``` After running the above users will have a set of files, each with 10000 lines. This ensures the memory overhead of the next command is not excessive (xml to JSON conversion is done in memory). ```bash find . -maxdepth 1 -type f -exec xq -c '.rows.row[]' {} \; | sed -e 's:"@:":g' > posts_v2.json ``` The above command will produce a single `posts.json` file. Load into ClickHouse with the following command. Note the schema is specified for the `posts.json` file. This will need to be adjusted per data type to align with the target table. ```bash clickhouse local --query "SELECT * FROM file('posts.json', JSONEachRow, 'Id Int32, PostTypeId UInt8, AcceptedAnswerId UInt32, CreationDate DateTime64(3, \'UTC\'), Score Int32, ViewCount UInt32, Body String, OwnerUserId Int32, OwnerDisplayName String, LastEditorUserId Int32, LastEditorDisplayName String, LastEditDate DateTime64(3, \'UTC\'), LastActivityDate DateTime64(3, \'UTC\'), Title String, Tags String, AnswerCount UInt16, CommentCount UInt8, FavoriteCount UInt8, ContentLicense String, ParentId String, CommunityOwnedDate DateTime64(3, \'UTC\'), ClosedDate DateTime64(3, \'UTC\')') FORMAT Native" | clickhouse client --host --secure --password --query "INSERT INTO stackoverflow.posts_v2 FORMAT Native" ``` ## Example queries A few simple questions to you get started. ### Most popular tags on Stack Overflow ```sql SELECT arrayJoin(arrayFilter(t -> (t != ''), splitByChar('|', Tags))) AS Tags, count() AS c FROM stackoverflow.posts GROUP BY Tags ORDER BY c DESC LIMIT 10 ┌─Tags───────┬───────c─┐ │ javascript │ 2527130 │ │ python │ 2189638 │ │ java │ 1916156 │ │ c# │ 1614236 │ │ php │ 1463901 │ │ android │ 1416442 │ │ html │ 1186567 │ │ jquery │ 1034621 │ │ c++ │ 806202 │ │ css │ 803755 │ └────────────┴─────────┘ 10 rows in set. Elapsed: 1.013 sec. Processed 59.82 million rows, 1.21 GB (59.07 million rows/s., 1.19 GB/s.) Peak memory usage: 224.03 MiB. ``` ### User with the most answers (active accounts) Account requires a `UserId`. ```sql SELECT any(OwnerUserId) UserId, OwnerDisplayName, count() AS c FROM stackoverflow.posts WHERE OwnerDisplayName != '' AND PostTypeId='Answer' AND OwnerUserId != 0 GROUP BY OwnerDisplayName ORDER BY c DESC LIMIT 5 ┌─UserId─┬─OwnerDisplayName─┬────c─┐ │ 22656 │ Jon Skeet │ 2727 │ │ 23354 │ Marc Gravell │ 2150 │ │ 12950 │ tvanfosson │ 1530 │ │ 3043 │ Joel Coehoorn │ 1438 │ │ 10661 │ S.Lott │ 1087 │ └────────┴──────────────────┴──────┘ 5 rows in set. Elapsed: 0.154 sec. Processed 35.83 million rows, 193.39 MB (232.33 million rows/s., 1.25 GB/s.) Peak memory usage: 206.45 MiB. ``` ### ClickHouse related posts with the most views ```sql SELECT Id, Title, ViewCount, AnswerCount FROM stackoverflow.posts WHERE Title ILIKE '%ClickHouse%' ORDER BY ViewCount DESC LIMIT 10 ┌───────Id─┬─Title────────────────────────────────────────────────────────────────────────────┬─ViewCount─┬─AnswerCount─┐ │ 52355143 │ Is it possible to delete old records from clickhouse table? │ 41462 │ 3 │ │ 37954203 │ Clickhouse Data Import │ 38735 │ 3 │ │ 37901642 │ Updating data in Clickhouse │ 36236 │ 6 │ │ 58422110 │ Pandas: How to insert dataframe into Clickhouse │ 29731 │ 4 │ │ 63621318 │ DBeaver - Clickhouse - SQL Error [159] .. Read timed out │ 27350 │ 1 │ │ 47591813 │ How to filter clickhouse table by array column contents? │ 27078 │ 2 │ │ 58728436 │ How to search the string in query with case insensitive on Clickhouse database? │ 26567 │ 3 │ │ 65316905 │ Clickhouse: DB::Exception: Memory limit (for query) exceeded │ 24899 │ 2 │ │ 49944865 │ How to add a column in clickhouse │ 24424 │ 1 │ │ 59712399 │ How to cast date Strings to DateTime format with extended parsing in ClickHouse? │ 22620 │ 1 │ └──────────┴──────────────────────────────────────────────────────────────────────────────────┴───────────┴─────────────┘ 10 rows in set. Elapsed: 0.472 sec. Processed 59.82 million rows, 1.91 GB (126.63 million rows/s., 4.03 GB/s.) Peak memory usage: 240.01 MiB. ``` ### Most controversial posts ```sql SELECT Id, Title, UpVotes, DownVotes, abs(UpVotes - DownVotes) AS Controversial_ratio FROM stackoverflow.posts INNER JOIN ( SELECT PostId, countIf(VoteTypeId = 2) AS UpVotes, countIf(VoteTypeId = 3) AS DownVotes FROM stackoverflow.votes GROUP BY PostId HAVING (UpVotes > 10) AND (DownVotes > 10) ) AS votes ON posts.Id = votes.PostId WHERE Title != '' ORDER BY Controversial_ratio ASC LIMIT 3 ┌───────Id─┬─Title─────────────────────────────────────────────┬─UpVotes─┬─DownVotes─┬─Controversial_ratio─┐ │ 583177 │ VB.NET Infinite For Loop │ 12 │ 12 │ 0 │ │ 9756797 │ Read console input as enumerable - one statement? │ 16 │ 16 │ 0 │ │ 13329132 │ What's the point of ARGV in Ruby? │ 22 │ 22 │ 0 │ └──────────┴───────────────────────────────────────────────────┴─────────┴───────────┴─────────────────────┘ 3 rows in set. Elapsed: 4.779 sec. Processed 298.80 million rows, 3.16 GB (62.52 million rows/s., 661.05 MB/s.) Peak memory usage: 6.05 GiB. ``` ## Attribution We thank Stack Overflow for providing this data under the `cc-by-sa 4.0` license, acknowledging their efforts and the original source of the data at [https://archive.org/details/stackexchange](https://archive.org/details/stackexchange).