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https://issues.apache.org/jira/browse/PHOENIX-4845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16721701#comment-16721701
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Daniel Wong commented on PHOENIX-4845:
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h2. SQL Standard and Phoenix-3383
The SQL99 standard states:
{quote}7) Let Rx and Ry be the two <row value constructor>s of the <com-
parison predicate> and let RXi and RYi be the i-th <row value
constructor element>s of Rx and Ry, respectively. "Rx <comp op>
Ry" is true, false, or unknown as follows:
...
"Rx > Ry" is true if and only if RXi = RYi for all i < n and
RXn > RYn for some n.
{quote}
Phoenix-3383 sought to properly provide SQL standard behavior as it was found
that the current behavior, which supported pagination did not match SQL
standard.
Consider tables with pk columns with differing sort orders.
||Row #||A Desc||B||
|1|e|1|
|2|d|2|
|3|c|1|
|4|c|2|
|5|c|3|
|6|b|1|
|7|a|2|
By SQL standard if the select was
{code:java}
SELECT * FROM TABLE ORDER BY A DESC, B WHERE (A, B) > ('c', 2)
{code}
We would expect rows 1,2, and 5 to return today we return rows 5,6,7. Note that
the rows are not contiguous and therefore RVC is not suited by SQL standard to
pagination. In order to merge Phoenix-3383 and support SQL standard we must
change how we perform pagination.
h1. Recommended Approaches
h2. PHOENIX-4845 Support using Row Value Constructors in OFFSET clause to
support paging in tables where the sort order of PK columns varies.
Provide syntax like:
{code:java}
SELECT * FROM TABLE ORDER BY A, B OFFSET (?,?) LIMIT 2000
{code}
The scope of this clause is for single table paginated selects with the row key
similar to the base table or index. Exceptions will be thrown if the following
happen: Order By does not match either an index or the base table, more than
one table is specified in the from clause, the RVC in the Offset clause does
not match the leading edge of the row keys.
The new syntax will trigger the Phoenix client to push the RVC key to the
start row of the HBase scan.
h2. Extend PHOENIX-2606 to support server side cursors.
Extend Phoenix Cursor syntax like:
{code:java}
DECLARE MY_TABLE_CURSOR CURSOR SERVER FOR SELECT * FROM TABLE
{code}
we would add a row to a new System Table, SYSTEM.CURSORS similar to the
following, with PK(CURSOR_NAME,CURSOR_INSTANCE_ID,TABLE)
||CURSOR_NAME||CURSOR_INSTANCE_ID||TABLE||QUERY||LAST_KEY||
|MY_TABLE_CURSOR|123|TABLE|SELECT * FROM TABLE|null|
| | | | | |
On the following fetches similar to :
{code:java}
FETCH NEXT 2000 ROWS FROM MY_TABLE_CURSOR
{code}
We would fetch from the system table, and construct a a query similar to the
stored query and use the LAST_KEY as the start row of the scan.
h2. Advantages and Disadvantages
h3. OFFSET advantages
Requires less code changes in Phoenix
Both cursor and offset require being able to push a row key into a scan
h3. OFFSET DISADVANTAGES
Offset is not SQL standard
h3. Server Side Cursor advantages
SQL standard behavior and intended method for pagination
As service owners being able to provide pagination easily to the end users
will provide benefits
Since the users' intent is clear for pagination, performance and behavior
improvements can be delivered without affecting users.
h3. Disadvantages
CURSOR is that it's stateful, now we need to keep track of the state and query
about that state.
Larger code changes required as well as management of more metadata.
h2. Proposal
Implement OFFSET first, in the future for Phoenix's general usability leveraged
it as a piece of future cursor implementations.
h1.
> Support using Row Value Constructors in OFFSET clause to support paging in
> tables where the sort order of PK columns varies
> ---------------------------------------------------------------------------------------------------------------------------
>
> Key: PHOENIX-4845
> URL: https://issues.apache.org/jira/browse/PHOENIX-4845
> Project: Phoenix
> Issue Type: New Feature
> Reporter: Thomas D'Silva
> Priority: Major
> Labels: DESC, SFDC
> Attachments: PHOENIX-offset.txt
>
>
> RVCs along with the LIMIT clause are useful for efficiently paging through
> rows (see [http://phoenix.apache.org/paged.html]). This works well if the pk
> columns are sorted ascending, we can always use the > operator to query for
> the next batch of row.
> However if the PK of a table is (A DESC, B DESC) we cannot use the following
> query to page through the data
> {code:java}
> SELECT * FROM TABLE WHERE (A, B) > (?, ?) ORDER BY A DESC, B DESC LIMIT 20
> {code}
> Since the rows are sorted by A desc and then by B descending we need change
> the comparison order
> {code:java}
> SELECT * FROM TABLE WHERE (A, B) < (?, ?) ORDER BY A DESC, B DESC LIMIT 20
> {code}
> If the PK of a table contains columns with mixed sort order for eg (A DESC,
> B) then we cannot use RVC to page through data.
> If we supported using RVCs in the offset clause we could use the offset to
> set the start row of the scan. Clients would not have to have logic to
> determine the comparison operator. This would also support paging through
> data for tables where the PK columns are sorted in mixed order.
> {code:java}
> SELECT * FROM TABLE ORDER BY A DESC, B LIMIT 20 OFFSET (?,?)
> {code}
> We would only allow using the offset if the rows are ordered by the sort
> order of the PK columns.
>
> FYI [~jfernando_sfdc]
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