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DOCAPI-5102: Description of insert_sample_with_metadata. Update of JSONEachRow docs. (#4653)
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BayoNet
Ivan Blinkov
parent
72a20d372e
commit
c39c32973c
@ -242,7 +242,7 @@ SELECT SearchPhrase, count() AS c FROM test.hits GROUP BY SearchPhrase WITH TOTA
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}
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```
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The JSON is compatible with JavaScript. To ensure this, some characters are additionally escaped: the slash `/` is escaped as `\/`; alternative line breaks `U+2028` and `U+2029`, which break some browsers, are escaped as `\uXXXX`. ASCII control characters are escaped: backspace, form feed, line feed, carriage return, and horizontal tab are replaced with `\b`, `\f`, `\n`, `\r`, `\t` , as well as the remaining bytes in the 00-1F range using `\uXXXX` sequences. Invalid UTF-8 sequences are changed to the replacement character <20> so the output text will consist of valid UTF-8 sequences. For compatibility with JavaScript, Int64 and UInt64 integers are enclosed in double quotes by default. To remove the quotes, you can set the configuration parameter output_format_json_quote_64bit_integers to 0.
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The JSON is compatible with JavaScript. To ensure this, some characters are additionally escaped: the slash `/` is escaped as `\/`; alternative line breaks `U+2028` and `U+2029`, which break some browsers, are escaped as `\uXXXX`. ASCII control characters are escaped: backspace, form feed, line feed, carriage return, and horizontal tab are replaced with `\b`, `\f`, `\n`, `\r`, `\t` , as well as the remaining bytes in the 00-1F range using `\uXXXX` sequences. Invalid UTF-8 sequences are changed to the replacement character <20> so the output text will consist of valid UTF-8 sequences. For compatibility with JavaScript, Int64 and UInt64 integers are enclosed in double quotes by default. To remove the quotes, you can set the configuration parameter [output_format_json_quote_64bit_integers](../operations/settings/settings.md#session_settings-output_format_json_quote_64bit_integers) to 0.
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`rows` – The total number of output rows.
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@ -257,7 +257,7 @@ This format is only appropriate for outputting a query result, but not for parsi
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ClickHouse supports [NULL](../query_language/syntax.md), which is displayed as `null` in the JSON output.
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See also the JSONEachRow format.
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See also the [JSONEachRow](#jsoneachrow) format.
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## JSONCompact {#jsoncompact}
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@ -307,24 +307,73 @@ See also the `JSONEachRow` format.
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## JSONEachRow {#jsoneachrow}
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Outputs data as separate JSON objects for each row (newline delimited JSON).
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When using this format, ClickHouse outputs rows as separated, newline delimited JSON objects, but the whole data is not a valid JSON.
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```json
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{"SearchPhrase":"","count()":"8267016"}
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{"SearchPhrase": "bathroom interior design","count()": "2166"}
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{"SearchPhrase":"yandex","count()":"1655"}
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{"SearchPhrase":"2014 spring fashion","count()":"1549"}
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{"SearchPhrase":"freeform photo","count()":"1480"}
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{"SearchPhrase":"angelina jolie","count()":"1245"}
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{"SearchPhrase":"omsk","count()":"1112"}
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{"SearchPhrase":"photos of dog breeds","count()":"1091"}
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{"SearchPhrase":"curtain designs","count()":"1064"}
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{"SearchPhrase":"baku","count()":"1000"}
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{"SearchPhrase":"","count":"8267016"}
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```
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Unlike the JSON format, there is no substitution of invalid UTF-8 sequences. Any set of bytes can be output in the rows. This is necessary so that data can be formatted without losing any information. Values are escaped in the same way as for JSON.
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When inserting the data, you should provide a separate JSON object for each row.
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For parsing, any order is supported for the values of different columns. It is acceptable for some values to be omitted – they are treated as equal to their default values. In this case, zeros and blank rows are used as default values. Complex values that could be specified in the table are not supported as defaults, but it can be turned on by option `insert_sample_with_metadata=1`. Whitespace between elements is ignored. If a comma is placed after the objects, it is ignored. Objects don't necessarily have to be separated by new lines.
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### Inserting the Data
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```
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INSERT INTO UserActivity FORMAT JSONEachRow {"PageViews":5, "UserID":"4324182021466249494", "Duration":146,"Sign":-1} {"UserID":"4324182021466249494","PageViews":6,"Duration":185,"Sign":1}
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```
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ClickHouse allows:
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- Any order of key-value pairs in the object.
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- The omission of some values.
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ClickHouse ignores spaces between elements and commas after the objects. You can pass all the objects to a line. You do not have to separate them with line breaks.
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**Processing of omitted values**
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ClickHouse substitutes the omitted values with the default values of corresponding [data types](../data_types/index.md).
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In case of the `DEFAULT expr` is specified, ClickHouse uses different substitution rules depending on the [insert_sample_with_metadata](../operations/settings/settings.md#session_settings-insert_sample_with_metadata) setting.
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Let's consider the following table:
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```
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CREATE TABLE IF NOT EXISTS example_table
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(
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x UInt32,
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a DEFAULT x * 2
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) ENGINE = Memory;
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```
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- If `insert_sample_with_metadata = 0`, then the default value for `x` and `a` equals `0` (as a default value for `UInt32` data type).
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- If `insert_sample_with_metadata = 1`, then the default value for `x` equals `0`, but the default value of `a` equals `x * 2`.
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!!! note "Warning"
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Use this option carefully, enabling it negatively affects the performance of the ClickHouse server.
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### Selecting the Data
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Let's consider the `UserActivity` table as an example:
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```
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┌──────────────UserID─┬─PageViews─┬─Duration─┬─Sign─┐
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│ 4324182021466249494 │ 5 │ 146 │ -1 │
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│ 4324182021466249494 │ 6 │ 185 │ 1 │
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└─────────────────────┴───────────┴──────────┴──────┘
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```
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The query `SELECT * FROM UserActivity FORMAT JSONEachRow` returns:
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```
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{"UserID":"4324182021466249494","PageViews":5,"Duration":146,"Sign":-1}
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{"UserID":"4324182021466249494","PageViews":6,"Duration":185,"Sign":1}
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```
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Unlike the [JSON](#json) format, there is no substitution of invalid UTF-8 sequences. Values are escaped in the same way as for `JSON`.
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!!! note "Note"
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Any set of bytes can be output in the strings. Use the `JSONEachRow` format if you are sure that the data in the table can be formatted into JSON without losing any information.
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## Native {#native}
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@ -81,9 +81,29 @@ If an error occurred while reading rows but the error counter is still less than
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If `input_format_allow_errors_ratio` is exceeded, ClickHouse throws an exception.
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## insert_sample_with_metadata
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## insert_sample_with_metadata {#session_settings-insert_sample_with_metadata}
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For INSERT queries, specifies that the server need to send metadata about column defaults to the client. This will be used to calculate default expressions. Disabled by default.
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Turns on/off the extended data exchange between a ClickHouse client and a ClickHouse server. The setting is applies for `INSERT` queries.
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When executing the `INSERT` query, ClickHouse client prepares data and sends it to the server for writing. During the preparation of the data, the client gets the table structure from the server. In some cases, the client needs more information than the server sends by default. Turn on the extended data exchange with `insert_sample_with_metadata = 1`.
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When the extended data exchange is enabled, the server sends the additional metadata along with the table structure. The composition of the metadata depends on the operation.
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Operations where you may need the extended data exchange enabled:
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- Inserting the data of the [JSONEachRow](../../interfaces/formats.md#jsoneachrow) format.
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For all other operations ClickHouse doesn't apply the setting.
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!!! note "Note"
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The functionality of the extended data exchange consumes additional computing resources on the server and can reduce the performance.
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**Possible values**
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- 0 — Functionality is disabled.
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- 1 — Functionality is enabled.
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**Default value:** 0.
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## join_default_strictness {#settings-join_default_strictness}
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@ -406,7 +426,7 @@ The results of compilation are saved in the build directory in the form of .so f
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If the value is true, running INSERT skips input data from columns with unknown names. Otherwise, this situation will generate an exception.
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It works for JSONEachRow and TSKV formats.
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## output_format_json_quote_64bit_integers
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## output_format_json_quote_64bit_integers {#session_settings-output_format_json_quote_64bit_integers}
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If the value is true, integers appear in quotes when using JSON\* Int64 and UInt64 formats (for compatibility with most JavaScript implementations); otherwise, integers are output without the quotes.
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