Since create_hypertable() allows you to optionally specify a
partitioning column, it makes sense to default to one partition when
no column is specified and asking for the number of partitions when a
column is specified and the number of partitions is not (instead of
defaulting to one).
This patch also changes the order and type of partitioning-related
input arguments to create_hypertable() so that the number of
partitions can easily be specified alongside the partitioning column
and without type casting.
Hypertables that have no partitioning column set will experience
a segmentation fault at query time when a partitioning info struct
is read without being properly initialized. This change will zero-
initialize the struct so that the partitioning column name will
be a NULL pointer instead of garbage if not set.
Having the testing instructions in the installation instructions
was confusing and error-prone due to having to set up your
postgresql.conf file. It is not a necessary step for installation.
The chunk catalog table is now scanned with a native
scan rather than SPI call.
The scanner module is also updated with the option of
of taking locks on found tuples. In the case of chunk
scanning, chunks are typically returned with a share
lock on the tuple.
Aborting scans can be useful when scanning large tables or indexes and
the scan function knows that a condition has been met (e.g., all
tuples needed have been found). For instance, when scanning for
hypertable partitions, one can abort the scan when the found tuple
count equals the number of partitions in the partition epoch.
This patch refactors the code to use native heap/index scans for
finding partition epochs and partitions. It also moves the
partitioning-related code and data structures to partitioning.{c,h}.
This is a first stab at moving from SPI queries
to direct heap/index scans for cleaner and more
efficient code. By doing direct scans, there is no
need to prepare and cache a bunch of query plans
that make the code both slower and more complex.
This patch also adds a catalog module that keeps
cached OIDs and other things for catalog tables.
The cached information is updated every time the
backend switches to a new database. A permission
check is also implemented when accessing the catalog
information, but should probably be extended to
tables and schemas in the future.
There are two reasons for adding the partition count to
the partition_epoch table:
* It makes the partition_epoch more self-describing as
it makes it easy to see how many partitions are
in the current epoch as well as past ones.
* It simplifies native code that can read the partition
epoch, allocate memory for the right number of partitions,
and finally scan the partition table filling in each entry.
This patch fixes two deadlock cases.
The first case occurred as a result of taking partition and chunk
locks in inconsistent orders. When creating the first chunk C1
in a table, concurrent INSERT workers would race to create
that chunk. The result would be that the transactions queue up on
the partition lock P, effectively serializing these transactions.
This would lead to these concurrent transactions to insert
at very different offsets in time, one at a time. At some point
in the future, some n'th transaction Tn queued up on P would get
that lock as the preceeding inserters T1-(n-1) finish their inserts
and move on to their next batches. When Tn finally holds P, one of
the preceeding workers starts a new transaction that finds that it
needs to close C1, grabbing a lock on C1 and then on P. However,
it will block on P since Tn already holds P. Tn will also believe
it needs to close C1, thus trying to grab a lock on C1, but will
block, causing a deadlock.
The second case can occur on multi-partition hypertables. With
multiple partitions there are more than one open-ended chunk
at a time (one for each partition). This leads to a deadlock case
when two processes try to close (and thus lock) the chunks in
different order. For instance process P1 closes chunk C1 and then
C2, while process P2 locks in order C2 and C1.
The fix for the first case is to remove the partition lock
altogether. As it turns out, this lock is not needed.
Instead, transactions can race to create new chunks, thus causing
conflicts. A conflict in creating a new chunk can safely be
ignored and it also avoids taking unecessary locks. Removing the
partition lock also avoids the transaction serialization that
happens around this lock, which is especially bad for long-running
transactions (e.g., big INSERT batches).
The fix for the second multi-partition deadlock case is to always
close chunks in chunk ID order. This requires closing chunks at
the end of a transaction, once a transaction knows all the chunks
it needs to close. This also has the added benefit of reducing the
time a transaction holds exclusive locks on chunks, potentially
improving insert performance.
Currently, the internal metadata tables for hypertables track time
as a BIGINT integer. Converting hypertable time columns in TIMESTAMP
format to this internal representation requires using Postgres' conversion
functions that are imprecise due to floating-point arithmetic. This patch
adds C-based conversion functions that offer the following conversions
using accurate integer arithmetic:
- TIMESTAMP to UNIX epoch BIGINT in microseconds
- UNIX epoch BIGINT in microseconds to TIMESTAMP
- TIMESTAMP to Postgres epoch BIGINT in microseconds
- Postgres epoch BIGINT in microseconds to TIMESTAMP
The downside of the UNIX epoch functions are that they don't offer the full
date range as offered by the Postgres to_timestamp() function. This is
because of the required epoch shift might otherwise overflow the BIGINT.
All functions should, however, offer appropriate range checks and will
throw errors if outside the range.
