Doc changes to account for correctness fixes for sequences

As a result of the correctness fixes made by DERBY-5493 and DERBY-5494, we should adjust the user documentation for sequences. I recommend the following changes:

1) Reference Guide

a) Add a section for a new system function. This function lets users observe the instantaneous current value of a sequence generator without having to query SYSSEQUENCES. Querying SYSSEQUENCES didn't actually return the current value, it only returned an upper bound on that value, viz., the end of the chunk of sequence values which had been pre-allocated but not actually used. The new function shows you the very next value which will be returned by a NEXT VALUE FOR clause. Users should never directly query SYSSEQUENCES because that will cause sequence generator concurrency to slow down to a crawl.

Syntax

BIGINT SYSCS_UTIL.SYSCS_PEEK_AT_SEQUENCE(IN SCHEMANAME VARCHAR(128), IN SEQUENCENAME VARCHAR(128))

As with all system functions, schema and object name arguments are case-sensitive.

Execute privileges

By default, all users have execute privileges on this function.

b) SYSSEQUENCES system table

The Contents description for SYSSEQUENCES.CURRENTVALUE needs to be changed to make a couple points: This is not the actual next value for the sequence generator. That value can be obtained by calling SYSCS_PEEK_AT_SEQUENCE(). SYSSEQUENCES.CURRENTVALUE holds the end of the range of values which have been pre-allocated in order to boost concurrency. Users should not directly query SYSSEQUENCES because that will slow down the performance of sequence generators. Instead, users should call SYSCS_PEEK_AT_SEQUENCE().

c) derby.language.sequence.preallocator

The default value of this property has changed from 20 to 100.

In addition, I think that we should point out that if the database is shut down gracefully, then Derby should not leak unused, preallocated values; they will be thrown away and the sequence generator will pick up where it left off, once the database re-boots. However, if the database comes down ungracefully, then unused, preallocated values will be leaked: when the database comes up again, the sequence generator will skip the unused, preallocated values; this will leave a gap between the last NEXT VALUE FOR (issued before the database came down ungracefully) and the next NEXT VALUE FOR (issued after the database re-boots).

2) Tuning Guide

Maybe we should add a small section explaining how to boost the concurrency of sequences:

a) Don't query SYSSEQUENCES directly. Instead, use SYSCS_PEEK_AT_SEQUENCE().

b) Adjusting derby.language.sequence.preallocator may boost your concurrency. However, you should pay special attention to bringing down the database gracefully in order to not leak unused, preallocated values. See the Reference Manual section on derby.language.sequence.preallocator.