Improve `JSONEachRowRowInputFormat` by skipping all remaining fields when all required fields are read #62210

lgbo-ustc · 2024-04-03T02:10:45Z

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Performance Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

...
When all required fields are read, skip all remaining fields directly which can save a lot of comparison.

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lgbo-ustc · 2024-04-03T02:13:27Z

some test results

select count(uid) from
file('test.json', 'JSONEachRow', 'uid Nullable(String), latency Nullable(Int32), _region Nullable(String), shard Nullable(Int32)') settings max_threads=2

before

1 row in set. Elapsed: 1.919 sec. Processed 4.26 million rows, 663.47 MB (2.22 million rows/s., 345.82 MB/s.)

after

1 row in set. Elapsed: 1.146 sec. Processed 4.26 million rows, 663.47 MB (3.71 million rows/s., 578.76 MB/s.)

robot-clickhouse-ci-1 · 2024-04-03T08:30:01Z

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antaljanosbenjamin

My only concern is with this improvement we might parse invalid JSONs without any issues, e.g. when after the last necessary column there is an invalid JSON field.

As by default input_format_allow_errors_num is 0, I think this might mean that some currently not parsable JSONs become parsable.

I would at least introduce a setting to be able to disable this.

What about other formats, where we can have extra columns? Shouldn't the same logic be implemented for them too?

Avogar · 2024-04-03T12:14:23Z

src/Processors/Formats/Impl/JSONEachRowRowInputFormat.cpp

@@ -186,6 +187,11 @@ void JSONEachRowRowInputFormat::readJSONObject(MutableColumns & columns)
            JSONUtils::skipColon(*in);
            readField(column_index, columns);
        }
+        if (seen_columns_count >= total_columns)
+        {
+            skipToUnescapedNextLineOrEOF(*in);


This is not correct, we need to skip not up to the next line or EOF but to the next } symbol that is not inside quotes and track brackets balance (actually, we also need to do it in syncAfterError), because we support any delimiter betweeen JSON objects.

For example:

select * from format(JSONEachRow, 'a UInt32', '{"a" : 42}, {"a" : 43}')

Also string fields can have \n, and it will break this logic:

select * from format(JSONEachRow, 'a UInt32', '{"a" : 42, "b" : "\n"}, {"a" : 43}')

So yes, we need to skip up to the end of JSON object

~~I will create a PR with fix for syncAfterError with proper data skipping~~

UPD: it's not so obvious as I though, because when some row is a broken JSON we cannot just skip all up to the end of JSON object as there might me no end (as it's broken). So here we just have the easies and best solution - skip up to new line delimiter as in most cases it will work.

But in case of valid JSON we definitely should skip up to the end of JSON object. The similar code how to do it fast can be found in readJSONObjectOrArrayPossiblyInvalid function in ReadHelpers

Avogar · 2024-04-03T12:17:02Z

Totally agree with @antaljanosbenjamin, it should be done under a setting. Also we have setting input_format_skip_unknown_fields that allows to skip unknown keys during parsing, it's enabled by default but we still need to check if it's disabled before skipping any data after all columns are parsed.

lgbo-ustc · 2024-04-07T01:02:37Z

@Avogar @antaljanosbenjamin The way to skip unused fields has been changed. I think it would have the same behavior as before in this way.

lgbo-ustc · 2024-04-08T08:07:11Z

lgbo-ustc · 2024-04-09T06:28:23Z

Hi @antaljanosbenjamin could you review this again? The latest code just skip all remaining fields without looking up the name_map. If the json is invalid, it could fail as before.

antaljanosbenjamin · 2024-04-10T10:45:26Z

src/Processors/Formats/Impl/JSONEachRowRowInputFormat.cpp

@@ -161,6 +162,12 @@ void JSONEachRowRowInputFormat::readJSONObject(MutableColumns & columns)
    for (size_t key_index = 0; advanceToNextKey(key_index); ++key_index)
    {
        StringRef name_ref = readColumnName(*in);
+        if (seen_columns_count >= total_columns)


We would skip the duplicated fields check in readField in case we read all columns. Considering that I think we should introduce a new setting to enable that.

antaljanosbenjamin

Apart from this tiny change, I am okay with this PR.

antaljanosbenjamin · 2024-04-11T08:28:16Z

src/Core/SettingsChangesHistory.h

@@ -89,6 +89,7 @@ static std::map<ClickHouseVersion, SettingsChangesHistory::SettingsChanges> sett
              {"ignore_drop_queries_probability", 0, 0, "Allow to ignore drop queries in server with specified probability for testing purposes"},
              {"lightweight_deletes_sync", 2, 2, "The same as 'mutation_sync', but controls only execution of lightweight deletes"},
              {"query_cache_system_table_handling", "save", "throw", "The query cache no longer caches results of queries against system tables"},
+              {"input_format_json_throw_on_duplicated_fields", false, true, "Throw an exception if there are duplicated fields in JSON object in JSON input formats. If disabled, the first field will be used"},


Suggested change

{"input_format_json_throw_on_duplicated_fields", false, true, "Throw an exception if there are duplicated fields in JSON object in JSON input formats. If disabled, the first field will be used"},

{"input_format_json_throw_on_duplicated_fields", true, true, "Throw an exception if there are duplicated fields in JSON object in JSON input formats. If disabled, the first field will be used"},

In my understanding previously we thrown an exception, so the previous value of this should true, not false.

Avogar · 2024-04-11T11:07:40Z

Actually, I am not sure about introducing setting input_format_json_throw_on_duplicated_fields, it will be enabled by default and so this new optimization will be disabled by default. But it's a good optimization and I want it to work by default. Maybe we can add a setting for this optimization instead? We will set it to true by default, add to settings changes history and add it to docs with a note that in some cases duplicated fields won't be checked. WDYT?

antaljanosbenjamin · 2024-04-11T12:28:31Z

As @Avogar you have much more experience in input formats and how to handle new optimizations, settings, etc, plus I also think this is a nice optimization, I am happy if we enable the optimization by default.

You also made me think again about the setting. And I think you are right: we should have a setting to enable the optimization and not to disable throwing on repeated fields, because right now if the first two fields have the same name, we will still throw an exception regardless of the value of the new setting.

Thanks for jumping in!

@lgbo-ustc My review seems to be a bit bad on this PR, sorry for that. I am still learning about formats and might miss things like this. Please let us know what do you think.

lgbo-ustc · 2024-04-12T00:11:11Z

Indeed I want to enable this optimization at default. This optimization is inspired by a case we meet recently, In our actual production applications, json strings with invalid format or duplicated fields are rare.

antaljanosbenjamin

Thanks for the really nice work!

antaljanosbenjamin · 2024-04-17T13:32:23Z

test_ttl_move/test.py::test_alter_with_merge_work[mt_work] is flaky
Stress test (asan) failed because of fault injection. Because of the lack of proper stack traces it is hard to fix properly. Save the stacktrace of thread waiting on failing AsyncLoader job #62719
Stress test (tsan) failed also because of the fault injection, figuring out how to fix it.

Merging this PR.

lgbo-ustc force-pushed the json_format_early_skip branch from b4d8f3a to 9b3bf4e Compare April 3, 2024 02:11

lgbo-ustc force-pushed the json_format_early_skip branch 2 times, most recently from d5ed872 to 4fd2fc8 Compare April 3, 2024 02:17

lgbo-ustc marked this pull request as ready for review April 3, 2024 02:21

lgbo-ustc marked this pull request as draft April 3, 2024 02:23

lgbo-ustc force-pushed the json_format_early_skip branch from 4fd2fc8 to 673912f Compare April 3, 2024 02:29

improve by skip to line end when all fields are read

cbedf8c

lgbo-ustc force-pushed the json_format_early_skip branch from 673912f to cbedf8c Compare April 3, 2024 02:30

lgbo-ustc marked this pull request as ready for review April 3, 2024 02:30

antaljanosbenjamin self-assigned this Apr 3, 2024

robot-clickhouse-ci-1 added the pr-performance Pull request with some performance improvements label Apr 3, 2024

antaljanosbenjamin reviewed Apr 3, 2024

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update

416b8fb

Merge remote-tracking branch 'origin/master' into json_format_early_skip

28de6d8

Merge remote-tracking branch 'origin/master' into json_format_early_skip

b3363e4

lgbo-ustc changed the title ~~Improve JSONEachRowRowInputFormat by skipping to the line end when all fields are read~~ Improve JSONEachRowRowInputFormat by skipping all fields when all fields are read Apr 9, 2024

lgbo-ustc changed the title ~~Improve JSONEachRowRowInputFormat by skipping all fields when all fields are read~~ Improve JSONEachRowRowInputFormat by skipping all fields when all remaining fields are read Apr 9, 2024

lgbo-ustc changed the title ~~Improve JSONEachRowRowInputFormat by skipping all fields when all remaining fields are read~~ Improve JSONEachRowRowInputFormat by skipping all remaining fields when all required fields are read Apr 9, 2024

antaljanosbenjamin reviewed Apr 10, 2024

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lgbo-ustc added 3 commits April 11, 2024 14:36

add settings

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Merge remote-tracking branch 'origin/master' into json_format_early_skip

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update

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antaljanosbenjamin requested changes Apr 11, 2024

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lgbo-ustc added 2 commits April 11, 2024 16:31

update SettingsChangesHistory

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add comments

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lgbo-ustc force-pushed the json_format_early_skip branch from 0e3a3dc to d1b80d7 Compare April 11, 2024 08:34

lgbo-ustc added 3 commits April 12, 2024 08:52

update settings

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Merge remote-tracking branch 'origin/master' into json_format_early_skip

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update test

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antaljanosbenjamin approved these changes Apr 12, 2024

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lgbo-ustc added 2 commits April 17, 2024 08:56

Merge remote-tracking branch 'origin/master' into json_format_early_skip

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fixed typos

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antaljanosbenjamin added this pull request to the merge queue Apr 17, 2024

Merged via the queue into ClickHouse:master with commit 6189cd4 Apr 17, 2024
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Improve `JSONEachRowRowInputFormat` by skipping all remaining fields when all required fields are read #62210

Improve `JSONEachRowRowInputFormat` by skipping all remaining fields when all required fields are read #62210

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antaljanosbenjamin commented Apr 17, 2024

	{"input_format_json_throw_on_duplicated_fields", false, true, "Throw an exception if there are duplicated fields in JSON object in JSON input formats. If disabled, the first field will be used"},
	{"input_format_json_throw_on_duplicated_fields", true, true, "Throw an exception if there are duplicated fields in JSON object in JSON input formats. If disabled, the first field will be used"},

Improve JSONEachRowRowInputFormat by skipping all remaining fields when all required fields are read #62210

Improve JSONEachRowRowInputFormat by skipping all remaining fields when all required fields are read #62210

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Modify your CI run:

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Extra options:

Only specified batches in multi-batch jobs:

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Improve `JSONEachRowRowInputFormat` by skipping all remaining fields when all required fields are read #62210

Improve `JSONEachRowRowInputFormat` by skipping all remaining fields when all required fields are read #62210

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