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Re: scrolling in ElasticSearch adapter


In JDBC one can configure a fetch size which would reflect the amount of rows to be fetched initially, but also subsequently. https://docs.oracle.com/javase/7/docs/api/java/sql/Statement.html#setFetchSize(int)

According to what you are writing, ES behvior is what TYPE_SCROLL_INSENSITIVE would do i.e. provide a snapshot view that isn't affected by changes.

IMO TYPE_SCROLL_SENSITIVE means that if you have rows R1, R2, R3, R4, ... and view R1, R2, then R3 is deleted and you fetch the next rows, you wouldn't see R3.

According to the JDBC spec (https://docs.oracle.com/javase/7/docs/api/java/sql/Connection.html#prepareStatement(java.lang.String,%20int,%20int) ) you don't have to support all modes. Usually, user code doesn't use scrolling that much, but at least forward scrolling makes sense.

Am 24.10.2018 um 21:38 schrieb Andrei Sereda:
Hi Julian,

Scrolling (in elastic) does not only mean “open a cursor” but also iterate
over consistent snapshot. From docs:

The results that are returned from a scroll request reflect the state of
the index at the time that the initial search request was made, like a
snapshot in time. Subsequent changes to documents (index, update or delete)
will only affect later search requests.

So pagination (fetch / offset) can’t exactly replicate this functionality.

The problem with scrolling (in elastic) is that it is expensive and can’t
(shouldn’t) be enabled it by default.

There is one more “issue”. Currently select * from elastic returns at most
10 rows (in calcite). This is consistent with elastic behaviour which
limits result set to 10 documents (unless size is specified). When
returning a cursor (eg. using JDBC TYPE_SCROLL_SENSITIVE
<https://docs.oracle.com/javase/7/docs/api/java/sql/ResultSet.html#TYPE_SCROLL_SENSITIVE>
or SQL hint) does it mean return whole elastic index ? I’m not at ease with
returning different results based on hints or cursor settings.

Andrei.

On Wed, Oct 24, 2018 at 3:02 PM Julian Hyde <jhyde.apache@xxxxxxxxx> wrote:

It seems to me that Elasticsearch scroll means return a cursor - a
collection of rows that you iterate over, and you may not read all of them.
This is the default operation of JDBC.

So, I guess we need to give the user a way to signal their intent to read
all rows versus only the first few. Oracle’s FIRST_ROWS and ALL_ROWS
hints[1] seem close to this. We would want the hints to be acted upon by
both the optimizer and the JDBC transport.

Related is pagination. SQL has FETCH and OFFSET, which allow you to
retrieve different pieces of a large result set in separate statements or
(using query parameters) executions. It would be useful if the server could
be given a hint to cache a statement across page requests.

Julian

[1]
https://docs.oracle.com/cd/B10500_01/server.920/a96533/hintsref.htm#4924

On Oct 24, 2018, at 11:19 AM, Christian Beikov <
christian.beikov@xxxxxxxxx> wrote:
Hey,

not sure if this should be an SQL keyword. JDBC specifies various
constants that can be used at statement creation time:
https://docs.oracle.com/javase/7/docs/api/java/sql/ResultSet.html
Not sure though if or how these configurations are accessible for data
stores or dialects, but IMO using these would be the proper way.
Regards

Christian

Am 24.10.2018 um 18:44 schrieb Andrei Sereda:
Hello,


I was thinking about adding [scrolling functionality](

https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-scroll.html
)
to elastic search adapter. Since scrolling has non-negligible effect on
the
cluster it should be selectively enabled on per query basis. So, likely,
user has to explicitly set "scroll flag" somewhere.

Most natural way seems in SQL. [Calcite sql grammar](
https://calcite.apache.org/docs/reference.html) has `SCROLL` keyword
(unused to my knowledge). There were also discussions about adding
hints to
Calcite.

### Examples
```sql
-- special sql keyword ?
SCROLL select * from elastic;

-- assuming hints are available in calcite
/* HINT: scroll */ select * from elastic;
```

What people think about this use-case ? Are there better ideas ?

Regards,
Andrei.