Analyzing Stack Overflow data with ClickHouse

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[​](#convert-to-json "Direct link to Convert to JSON")

At the time of writing, ClickHouse doesn't 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:

```
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.

```
p7zip -d stackoverflow.com-Posts.7z
```

The following splits the xml file into files, each containing 10000 rows.

```
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 "<rows>\n"; cat - ; printf "</rows>\n"; } > $FILE' -
```

After running the above you will have a set of files, each with 10000 lines. This ensures the memory overhead of the next command isn't excessive (xml to JSON conversion is done in memory).

```
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.

```
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 <host> --secure --password <password> --query "INSERT INTO stackoverflow.posts_v2 FORMAT Native"
```

## Example queries[​](#example-queries "Direct link to Example queries")

A few simple questions to you get started.

### Most popular tags on Stack Overflow[​](#most-popular-tags-on-stack-overflow "Direct link to Most popular tags on Stack Overflow")

```
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)[​](#user-with-the-most-answers-active-accounts "Direct link to User with the most answers (active accounts)")

Account requires a `UserId`.

```
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[​](#clickhouse-related-posts-with-the-most-views "Direct link to ClickHouse related posts with the most views")

```
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[​](#most-controversial-posts "Direct link to Most controversial posts")

```
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[​](#attribution "Direct link to 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>.