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https://issues.apache.org/jira/browse/CALCITE-6909?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17938417#comment-17938417
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Yu Xu commented on CALCITE-6909:
--------------------------------
Very thanks for [~julianhyde] brilliant suggestion! this issue is aim to fix
the bug that ClickHouse Dialect can not support accurate precision.
I test it in ClickHouse again according to the CK doc and seek advice from some
ClickHouse experts at the same time,
I had a mistake about it, Decimal(P, S) can hold all condition with
Decimal32/64/128/256(these just a synonym of Decimal(P, S), and I miss this
important message). IMO we just need to resolve the precision problem in
ClickHouse dialect:ClickHouse max precision is 76, but in dialect just use
default max value which is 19, that can not support precision higher than 19.
I had updated the summary/description and changed the fix way in related PR, if
found any problem please feedback anytime, thanks again to [~julianhyde]
[~nobigo]
> ClickHouse Dailect can not support accurate precision
> -----------------------------------------------------
>
> Key: CALCITE-6909
> URL: https://issues.apache.org/jira/browse/CALCITE-6909
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.39.0
> Reporter: Yu Xu
> Assignee: Yu Xu
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.40.0
>
>
> Currently ClickHouse Dailect can not support accurate precision
> In ClickHouse:
> * *P - precision. Valid range: [ 1 : 76 ].*
> * *S - scale. Valid range: [ 0 : P ].*
> Depending on P parameter value Decimal(P, S) is a synonym for:
> * *P from [ 1 : 9 ] - for Decimal32(S)*
> * *P from [ 10 : 18 ] - for Decimal64(S)*
> * *P from [ 19 : 38 ] - for Decimal128(S)*
> * *P from [ 39 : 76 ] - for Decimal256(S)*
> we can refer to doc with:
> [https://clickhouse.com/docs/sql-reference/data-types/decimal]
>
> eg：{*}select cast(\"product_id\" as decimal(22,20)) from \"product\"{*}
> but transform to: *SELECT CAST(`product_id` AS DECIMAL(19, 19)) FROM
> `foodmart`.`product`*
>
> this should be transform to: *SELECT CAST(`product_id` AS DECIMAL(22, 20))
> FROM `foodmart`.`product`*
> because CK precision is from[1, 79]
>
> *user case:*
> if user want to cast to a decimal type such as :
> select cast('1.322131231313123311312322' as DECIMAL(30,24)) from
> system.numbers limit 1;
> should be a decimal number of 1.322131231313123311312322
> but currently calcite ClickHouse dialect can only cast
> to：1.322131231313123311
> this would cause loss of precision.
>
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