Support SELECT .. INTO OUTFILE ..TRUNCATE| APPEND..

[jovezhong]

In recent version of ClickHouse, there are settings for INTO OUTFILE to specify when the outfile exists, whether to append to clean up content and add.

https://clickhouse.com/docs/en/sql-reference/statements/select/into-outfile

When writing to a file that already exists, APPEND or TRUNCATE must be used.

However this is not available in Timeplus Proton.

Code: 62. DB::Exception: Syntax error: failed at position 1370 ('TRUNCATE') (line 31, col 56): TRUNCATE FORMAT LineAsString
. Expected one of: COMPRESSION, FORMAT, SETTINGS, end of query. (SYNTAX_ERROR)

The use case is to follow the ClickHouse docs script to generate the markdown based on settings source code.

SELECT prefix || (SELECT group_concat(*) FROM main_content)
INTO OUTFILE 'docs/en/operations/settings/settings.md' TRUNCATE FORMAT LineAsString

[jovezhong]

Oops, if I remove the TRUNCATE before the FORMAT LineAsString, the DB exception shows INTO OUTFILE is not allowed, but I remember this works before

Received exception from server (version 1.6.6):
Code: 358. DB::Exception: Received from localhost:8463. DB::Exception: INTO OUTFILE is not allowed. (INTO_OUTFILE_NOT_ALLOWED)