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https://issues.apache.org/jira/browse/DRILL-3534?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16137129#comment-16137129
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Paul Rogers commented on DRILL-3534:
------------------------------------
As John noted, the easiest way to envision {{INSERT INTO}} for big data would
be to add new files to an existing directory.
Fortunately for Drill, we can learn from existing solution such as
[Hive|https://cwiki.apache.org/confluence/display/Hive/LanguageManual+DML#LanguageManualDML-InsertingvaluesintotablesfromSQL]
and
[Impala|https://www.cloudera.com/documentation/enterprise/5-8-x/topics/impala_insert.html].
We should certainly learn from the strengths of those solutions, while trying
to avoid any pitfalls.
At the data level, two variations are possible:
* For file formats that allow it, Drill might append to the file. (CSV, TSV,
JSON for example.) (But, note that HDFS does not allow append operations though
MFS and Linux do.)
* For file formats that do not allow appending (e.g. Parquet), then adding
files to partition directories is a solution.
Drill is schema-on-read, which means that Drill, in general, does not know the
schema of data until it "looks." For a CSV file, this means we don't know the
file schema. (Although, for CSV files with headers, Drill could read the header
to approximate the schema.) So, it may be that it falls to the user to be
responsible to append only data that fits the existing schema. Since Drill (but
not its clients) handle schema change, Drill can deal with (some) files that
change schemas in the middle of the file, though Tableau and similar xDBC tools
cannot.
The difficulties emerge, however, at the physical level: dealing with
concurrency in a distributed, multi-user system. A key contribution of
databases is that they have kernels that deal with this physical layer (and do
such a great job that most of us never have to think about the issues.)
* When appending to a file, what do readers see? Do readers see a possibly
incomplete last record? Do they see an atomic update? (All the inserts or none?)
* What is the guarantee of consistency? What is the expected behavior if a
Drillbit crashes during an INSERT? OK to leave a file half written, or must
inserts be atomic?
* What happens if two queries try to insert into the same file? How are inserts
synchronized? (Or, does the last update win? Or, can files simply be
overwritten and it is up to the user to coordinate writes?)
* If the insert involves metadata (Parquet metadata, say) how are the metadata
updates synchronized with the INSERTs to provide a consistent view to readers?
* If an INSERT creates multiple files (as part of a partition directory), what
are the semantics for readers?
These issues are not at all at the logical SQL level; they are at the physical
level: dealing with the reality of a concurrent system.
The problems are made more complex because file systems do not provide the
semantics needed for DB-level synchronization. This is why the world needs
Oracle (and MySQL and Postgres): one cannot easily build DB semantics just
using file system mechanisms. (Though, many have tried.)
Possible approaches:
* The traditional solution is to use locking, transactions and the like to
synchronize work. These are (likely) not available to a query engine such as
Drill.
* Leverage a meta-store (such as Hive) to handle the synchronization.
* Leverage ZK to handle synchronization of just the insert/read operations
(e.g. place a marker in ZK to "lock" a file for updates.)
* Design a log-based approach, based on multi-version concurrency control
(MVCC) that allows writers to create new versions of files while readers read
old versions. The log file could be the coordination mechanism. An update is
not visible until the required entries appear in the log. There is a large
literature about using logs for synchronization.
Let the brainstorming begin!
> Insert into table support
> -------------------------
>
> Key: DRILL-3534
> URL: https://issues.apache.org/jira/browse/DRILL-3534
> Project: Apache Drill
> Issue Type: New Feature
> Reporter: Mehant Baid
> Fix For: Future
>
>
> Umbrella JIRA to track the "Insert into table" feature. More details
> regarding the scope, design etc will follow as things start to materialize.
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