Update links that resulted in redirects. Most are changes from http to
https, but there are also some other minor edits. (There are still some
redirects where the target URL looks less elegant than the one we
currently have. I have left those as is.)
In final_cost_hashjoin(), commit 9c7f5229a allowed inner_unique cases
to follow a code path previously used only for SEMI/ANTI joins; but it
neglected to fix an if-test within that path that assumed SEMI and ANTI
were the only possible cases. This resulted in a wrong value for
hashjointuples, and an ensuing bad cost estimate, for inner_unique normal
joins. Fortunately, for inner_unique normal joins we can assume the number
of joined tuples is the same as for a SEMI join; so there's no need for
more code, we just have to invert the test to check for ANTI not SEMI.
It turns out that in two contrib tests in which commit 9c7f5229a
changed the plan expected for a query, the change was actually wrong
and induced by this estimation error, not by any real improvement.
Hence this patch also reverts those changes.
Per report from RK Korlapati. Backpatch to v10 where the error was
introduced.
David Rowley
Discussion: https://postgr.es/m/CA+SNy03bhq0fodsfOkeWDCreNjJVjsdHwUsb7AG=jpe0PtZc_g@mail.gmail.com
lca_inner() wasn't prepared for the possibility of getting no inputs.
Fix that, and make some cosmetic improvements to the code while at it.
Also, I thought the documentation of this function as returning the
"longest common prefix" of the paths was entirely misleading; it really
returns a path one shorter than the longest common prefix, for the typical
definition of "prefix". Don't use that term in the docs, and adjust the
examples to clarify what really happens.
This has been broken since its beginning, so back-patch to all supported
branches.
Per report from Hailong Li. Thanks to Pierre Ducroquet for diagnosing
and for the initial patch, though I whacked it around some and added
test cases.
Discussion: https://postgr.es/m/5b0d8e4f-f2a3-1305-d612-e00e35a7be66@qunar.com
Now that the documentation sources are in XML rather than SGML, some of
the documentation about the editor, or more specifically Emacs, setup
needs updating. The updated instructions recommend using nxml-mode,
which works mostly out of the box, with some small tweaks in
emacs.samples and .dir-locals.el.
Also remove some obsolete stuff in .dir-locals.el. I did, however,
leave the sgml-mode settings in there so that someone using Emacs
without emacs.samples gets those settings when editing a *.sgml file.
As of commit 37a795a60, populate_recordset_worker() tried to pass back
(as rsi.setDesc) a tupdesc that it also had cached in its fn_extra.
But the core executor would free the passed-back tupdesc, risking a
crash if the function were called again in the same query. The safest
and least invasive way to fix that is to make an extra tupdesc copy
to pass back.
While at it, I failed to resist the temptation to get rid of unnecessary
get_fn_expr_argtype() calls here and in populate_record_worker().
Per report from Dmitry Dolgov; thanks to Michael Paquier and
Andrew Gierth for investigation and discussion.
Discussion: https://postgr.es/m/CA+q6zcWzN9ztCfR47ZwgTr1KLnuO6BAY6FurxXhovP4hxr+yOQ@mail.gmail.com
The patch that ended up as commit 3de241dba8 ("Foreign keys on
partitioned tables") lacked pg_dump tests, so the pg_dump code that was
there to support it inadvertently stopped working when in later
development I modified the backend code not to emit pg_trigger rows for
the partitioned table itself.
Bug analysis and code fix is by Michaël. I (Álvaro) added the test.
Reported-by: amul sul <sulamul@gmail.com>
Co-authored-by: Michaël Paquier <michael@paquier.xyz>
Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/CAAJ_b94n=UsNVhgs97vCaWEZAMe-tGDRVuZ73oePQH=eaJKGSA@mail.gmail.com
Previously convert_tuples_by_name_map naively performed a search of each
outdesc column starting at the first column in indesc and searched each
indesc column until a match was found. When partitioned tables had many
columns this could result in slow generation of the tuple conversion maps.
For INSERT and UPDATE statements that touched few rows, this could mean a
very large overhead indeed.
We can do a bit better with this loop. It's quite likely that the columns
in partitioned tables and their partitions are in the same order, so it
makes sense to start searching for each column outer column at the inner
column position 1 after where the previous match was found (per idea from
Alexander Kuzmenkov). This makes the best case search O(N) instead of
O(N^2). The worst case is still O(N^2), but it seems unlikely that would
happen.
Likewise, in the planner, make_inh_translation_list's search for the
matching column could often end up falling back on an O(N^2) type search.
This commit also improves that by first checking the column that follows
the previous match, instead of the column with the same attnum. If we
fail to match here we fallback on the syscache's hashtable lookup.
Author: David Rowley
Reviewed-by: Alexander Kuzmenkov
Discussion: https://www.postgresql.org/message-id/CAKJS1f9-wijVgMdRp6_qDMEQDJJ%2BA_n%3DxzZuTmLx5Fz6cwf%2B8A%40mail.gmail.com
When reading an existing FSM or VM page that was found to be corrupt by the
buffer manager, the code applied PageInit() to reinitialize the page, but
did so without any locking. There is thus a hazard that two backends might
concurrently do PageInit, which in itself would still be OK, but the slower
one might then zero over subsequent data changes applied by the faster one.
Even that is unlikely to be fatal; but it's not desirable, so add locking
to prevent it.
This does not add any locking overhead in the normal code path where the
page is OK. It's not immediately obvious that that's safe, but I believe
it is, for reasons explained in the added comments.
Problem noted by R P Asim. It's been like this for a long time, so
back-patch to all supported branches.
Discussion: https://postgr.es/m/CANXE4Te4G0TGq6cr0-TvwP0H4BNiK_-hB5gHe8mF+nz0mcYfMQ@mail.gmail.com
Starting and aborting transactions in security definer procedures
doesn't work. StartTransaction() insists that the security context
stack is empty, so this would currently cause a crash, and
AbortTransaction() resets it. This could be made to work by
reorganizing the code, but right now we just prohibit it.
Reported-by: amul sul <sulamul@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAAJ_b96Gupt_LFL7uNyy3c50-wbhA68NUjiK5%3DrF6_w%3Dpq_T%3DQ%40mail.gmail.com
All attributes and arguments using a slot name map to the data type
"name", but this function has been using "text". This is cosmetic, as
even if text is used then the slot name would be truncated to 64
characters anyway and stored as such. The documentation already said
so and the function already assumed that the argument was of this type
when fetching its value.
Bump catalog version.
Author: Sawada Masahiko
Discussion: https://postgr.es/m/CAD21AoADYz_-eAqH5AVFaCaojcRgwpo9PW=u8kgTMys63oB8Cw@mail.gmail.com
Temporary WAL segments are created in pg_wal and named as xlogtemp.pid
before being renamed to the real deal when creating a new segment. If
an instance crashes after the temporary segment is created and before
the rename is done, then the server would finish with unremovable data.
After an instance crash, scan pg_wal and remove any such segments. With
repetitive unlucky crashes this would contribute to disk bloat and
presents risks of ENOSPC especially with max_wal_size close to the
maximum allowed.
Author: Michael Paquier
Reviewed-by: Yugo Nagata, Heikki Linnakangas
Discussion: https://postgr.es/m/20180514054955.GF1528@paquier.xyz
In ad9a274778, shmem_exit_inprogress was
introduced. But we need to reset it after shmem_exit(), because unlike
the similar proc_exit(), shmem_exit() can also be called for cleanup
when the process will not exit.
Reported-by: Andrew Gierth <andrew@tao11.riddles.org.uk>
Explain that you can use any integer expression for the "count" in
pl/pgsql's versions of FETCH/MOVE, unlike the SQL versions which only
allow a constant.
Remove the duplicate version of this para under MOVE. I don't see
a good reason to maintain two identical paras when we just said that
MOVE works exactly like FETCH.
Per Pavel Stehule, though I didn't use his text.
Discussion: https://postgr.es/m/CAFj8pRAcvSXcNdUGx43bOK1e3NNPbQny7neoTLN42af+8MYWEA@mail.gmail.com
When truncating a table that is referenced by foreign keys in
partitioned tables, the check to ensure the referencing table are also
truncated spuriously failed. This is because it was relying on
relhastriggers as a proxy for the table having FKs, and that's wrong for
partitioned tables. Fix it to consider such tables separately. There
may be a better way ... but this code is pretty inefficient already.
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Michael Paquiër <michael@paquier.xyz>
Discussion: https://postgr.es/m/20180711000624.zmeizicibxeehhsg@alvherre.pgsql
Commit ff4f88916 adjusted the code to enforce the SQL spec's requirement
that a window using GROUPS mode must have an ORDER BY clause. But I missed
that the documentation explicitly said you didn't have to have one.
Also minor wordsmithing in the window-function section of select.sgml.
Per Masahiko Sawada, though I didn't use his patch.
An update that causes the tuple to be moved to a different partition was
missing out on re-constructing the to-be-updated tuple, based on the latest
tuple in the update chain. Instead, it's simply deleting the latest tuple
and inserting a new tuple in the new partition based on the old tuple.
Commit 2f17844104 didn't consider this case, so some of the updates were
getting lost.
In passing, change the argument order for output parameter in ExecDelete
and add some commentary about it.
Reported-by: Pavan Deolasee
Author: Amit Khandekar, with minor changes by me
Reviewed-by: Dilip Kumar, Amit Kapila and Alvaro Herrera
Backpatch-through: 11
Discussion: https://postgr.es/m/CAJ3gD9fRbEzDqdeDq1jxqZUb47kJn+tQ7=Bcgjc8quqKsDViKQ@mail.gmail.com
When it comes to SELECT ... FOR or LOCK, NOWAIT means to not wait for
something to happen, and issue an error. SKIP LOCKED means to not wait
for something to happen but to move on without issuing an error. The
internal option of autovacuum and autoanalyze mentioned above, used only
when wraparound is not involved was named NOWAIT, but behaves like SKIP
LOCKED which is confusing.
Author: Nathan Bossart
Discussion: https://postgr.es/m/20180307050345.GA3095@paquier.xyz
The code path where the assertion is added helps to check that
autovacuum always includes a relation OID when doing a vacuum on it.
Extracted from a larger patch set to add support for SKIP LOCKED with
manual VACUUM commands.
Author: Nathan Bossart
Discussion: https://postgr.es/m/9EF7EBE4-720D-4CF1-9D0E-4403D7E92990@amazon.com
WAL senders sending logically-decoded data fail to properly report in
"streaming" state when starting up, hence as long as one extra record is
not replayed, such WAL senders would remain in a "catchup" state, which
is inconsistent with the physical cousin.
This can be easily reproduced by for example using pg_recvlogical and
restarting the upstream server. The TAP tests have been slightly
modified to detect the failure and strengthened so as future tests also
make sure that a node is in streaming state when waiting for its
catchup.
Backpatch down to 9.4 where this code has been introduced.
Reported-by: Sawada Masahiko
Author: Simon Riggs, Sawada Masahiko
Reviewed-by: Petr Jelinek, Michael Paquier, Vaishnavi Prabakaran
Discussion: https://postgr.es/m/CAD21AoB2ZbCCqOx=bgKMcLrAvs1V0ZMqzs7wBTuDySezTGtMZA@mail.gmail.com
Generally, if the comparison operators for a datatype or pair of datatypes
are leakproof, the corresponding btree comparison support function can be
considered so as well. But we had not originally worried about marking
support functions as leakproof, reasoning that they'd not likely be used in
queries so the marking wouldn't matter. It turns out there's at least one
place where it does matter: calc_arraycontsel() finds the target datatype's
default btree comparison function and tries to use that to estimate
selectivity, but it will be blocked in some cases if the function isn't
leakproof. This leads to unnecessarily poor selectivity estimates and bad
plans, as seen in bug #15251.
Hence, run around and apply proleakproof markings where the corresponding
btree comparison operators are leakproof. (I did eyeball each function
to verify that it wasn't doing anything surprising, too.)
This isn't a full solution to bug #15251, and it's not back-patchable
because of the need for a catversion bump. A more useful response probably
is to consider whether we can check permissions on the parent table instead
of the child. However, this change will help in some cases where that
won't, and it's easy enough to do in HEAD, so let's do so.
Discussion: https://postgr.es/m/3876.1531261875@sss.pgh.pa.us
We should only run apply_pathtarget_labeling_to_tlist if CP_LABEL_TLIST
was specified, because only in that case has use_physical_tlist checked
that the labeling will succeed; otherwise we may get an "ORDER/GROUP BY
expression not found in targetlist" error. (This subsumes the previous
test about gating_clauses, because we reset "flags" to zero earlier
if there are gating clauses to apply.)
The only known case in which a failure can occur is with a ProjectSet
path directly atop a table scan path, although it seems likely that there
are other cases or will be such in future. This means that the failure
is currently only visible in the v10 branch: 9.6 didn't have ProjectSet,
while in v11 and HEAD, apply_scanjoin_target_to_paths for some weird
reason is using create_projection_path not apply_projection_to_path,
masking the problem because there's a ProjectionPath in between.
Nonetheless this code is clearly wrong on its own terms, so back-patch
to 9.6 where this logic was introduced.
Per report from Regina Obe.
Discussion: https://postgr.es/m/001501d40f88$75186950$5f493bf0$@pcorp.us
nodeWindowAgg.c failed to cope with the possibility that no ordering
columns are defined in the window frame for GROUPS mode or RANGE OFFSET
mode, leading to assertion failures or odd errors, as reported by Masahiko
Sawada and Lukas Eder. In RANGE OFFSET mode, an ordering column is really
required, so add an Assert about that. In GROUPS mode, the code would
work, except that the node initialization code wasn't in sync with the
execution code about when to set up tuplestore read pointers and spare
slots. Fix the latter for consistency's sake (even though I think the
changes described below make the out-of-sync cases unreachable for now).
Per SQL spec, a single ordering column is required for RANGE OFFSET mode,
and at least one ordering column is required for GROUPS mode. The parser
enforced the former but not the latter; add a check for that.
We were able to reach the no-ordering-column cases even with fully spec
compliant queries, though, because the planner would drop partitioning
and ordering columns from the generated plan if they were redundant with
earlier columns according to the redundant-pathkey logic, for instance
"PARTITION BY x ORDER BY y" in the presence of a "WHERE x=y" qual.
While in principle that's an optimization that could save some pointless
comparisons at runtime, it seems unlikely to be meaningful in the real
world. I think this behavior was not so much an intentional optimization
as a side-effect of an ancient decision to construct the plan node's
ordering-column info by reverse-engineering the PathKeys of the input
path. If we give up redundant-column removal then it takes very little
code to generate the plan node info directly from the WindowClause,
ensuring that we have the expected number of ordering columns in all
cases. (If anyone does complain about this, the planner could perhaps
be taught to remove redundant columns only when it's safe to do so,
ie *not* in RANGE OFFSET mode. But I doubt anyone ever will.)
With these changes, the WindowAggPath.winpathkeys field is not used for
anything anymore, so remove it.
The test cases added here are not actually very interesting given the
removal of the redundant-column-removal logic, but they would represent
important corner cases if anyone ever tries to put that back.
Tom Lane and Masahiko Sawada. Back-patch to v11 where RANGE OFFSET
and GROUPS modes were added.
Discussion: https://postgr.es/m/CAD21AoDrWqycq-w_+Bx1cjc+YUhZ11XTj9rfxNiNDojjBx8Fjw@mail.gmail.com
Discussion: https://postgr.es/m/153086788677.17476.8002640580496698831@wrigleys.postgresql.org
We include <float.h> in every place that needs isnan(), because MSVC
used to require it. However, since MSVC 2013 that's no longer necessary
(cf. commit cec8394b5c), so we can retire the inclusion to a
version-specific stanza in win32_port.h, where it doesn't need to
pollute random .c files. The header is of course still needed in a few
places for other reasons.
I (Álvaro) removed float.h from a few more files than in Emre's original
patch. This doesn't break the build in my system, but we'll see what
the buildfarm has to say about it all.
Author: Emre Hasegeli
Discussion: https://postgr.es/m/CAE2gYzyc0+5uG+Cd9-BSL7NKC8LSHLNg1Aq2=8ubjnUwut4_iw@mail.gmail.com
Header comment of shm_mq.c was mistakenly specifying path to shm_mq.h.
It was introduced in ec9037df. So, theoretically it could be
backpatched to 9.4, but it doesn't seem to worth it.
Such replication slots are physical slots freshly created without WAL
being reserved, which is the default behavior, which have not been used
yet as WAL consumption resources to retain WAL. This prevents advancing
a slot to a position older than any WAL available, which could falsify
calculations for WAL segment recycling.
This also cleans up a bit the code, as ReplicationSlotRelease() would be
called on ERROR, and improves error messages.
Reported-by: Kyotaro Horiguchi
Author: Michael Paquier
Reviewed-by: Andres Freund, Álvaro Herrera, Kyotaro Horiguchi
Discussion: https://postgr.es/m/20180626071305.GH31353@paquier.xyz
We fail to handle polymorphic types properly when they are used as
partition keys: we were unnecessarily adding a RelabelType node on top,
which confuses code examining the nodes. In particular, this makes
predtest.c-based partition pruning not to work, and ruleutils.c to emit
expressions that are uglier than needed. Fix it by not adding RelabelType
when not needed.
In master/11 the new pruning code is separate so it doesn't suffer from
this problem, since we already fixed it (in essentially the same way) in
e5dcbb88a1, which also added a few tests; back-patch those tests to
pg10 also. But since UPDATE/DELETE still uses predtest.c in pg11, this
change improves partitioning for those cases too. Add tests for this.
The ruleutils.c behavior change is relevant in pg11/master too.
Co-authored-by: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/54745d13-7ed4-54ac-97d8-ea1eec95ae25@lab.ntt.co.jp
PostgreSQL nowadays offers some kind of dynamic shared memory feature on
all supported platforms. Having the choice of "none" prevents us from
relying on DSM in core features. So this patch removes the choice of
"none".
Author: Kyotaro Horiguchi <horiguchi.kyotaro@lab.ntt.co.jp>
The EEOP_INNER_SYSVAR and EEOP_OUTER_SYSVAR executor opcodes are not
exercised by normal queries, because setrefs.c will resolve the references
to system columns in the scan nodes already. Join nodes refer to them by
their position in the child node's target list, like user columns.
The only place where those opcodes are used, is in evaluating a trigger's
WHEN condition that references system columns. Trigger evaluation abuses
the INNER/OUTER Vars to refer to the OLD and NEW tuples. The code to handle
the opcodes is pretty straightforward, but it seems like a good idea to
have some test coverage for them, anyway, so that they don't get removed or
broken by accident.
Author: Ashutosh Bapat, with some changes by me.
Discussion: https://www.postgresql.org/message-id/CAFjFpRerUFX=T0nSnCoroXAJMoo-xah9J+pi7+xDUx86PtQmew@mail.gmail.com
This is an option consistent with what pg_dump and pg_basebackup provide
which is useful for leveraging the I/O effort when testing things, not
to be used in a production environment.
Author: Michael Paquier
Reviewed-by: Heikki Linnakangas
Discussion: https://postgr.es/m/20180325122607.GB3707@paquier.xyz
The previous sync logic relied on looking for and then launching
externally initdb -S, which is a simple wrapper on top of fsync_pgdata.
There is nothing preventing pg_rewind to directly call this routine, so
remove the dependency to initdb and just call it directly.
Author: Michael Paquier
Reviewed-by: Heikki Linnakangas
Discussion: https://postgr.es/m/20180325122607.GB3707@paquier.xyz
Commit fafa374f2 caused _bt_getbuf() to possibly emit a WAL record for
a page that it was about to recycle. However, it failed to distinguish
all-zero pages from dead pages, which is important because only the
latter have valid btpo.xact values, or indeed any special space at all.
Recycling an all-zero page with XLogStandbyInfoActive() enabled therefore
led to an Assert failure, or to emission of a WAL record containing a
bogus cutoff XID, which might lead to unnecessary query cancellations
on hot standby servers.
Per reports from Antonin Houska and 自己. Amit Kapila was first to
propose this fix, and Robert Haas, myself, and Kyotaro Horiguchi
reviewed it at various times.
This is an old bug, so back-patch to all supported branches.
Discussion: https://postgr.es/m/2628.1474272158@localhost
Discussion: https://postgr.es/m/48875502.f4a0.1635f0c27b0.Coremail.zoulx1982@163.com
Commit 716ea626a attempted to fix the problem of building 1-D zero-size
arrays once and for all. But it turns out that contrib/intarray has some
code that doesn't use construct_array() but just builds arrays by hand,
so it didn't get the memo. This appears to affect all of subarray(),
intset_subtract(), inner_int_union(), inner_int_inter(), and
intarray_concat_arrays().
Back-patch into v11. In the past we've not back-patched this type of
change, but since v11 is still in beta it seems all right to include
this fix in it. Besides it's more consistent to make the fix in v11
where 716ea626a appeared.
Report and patch by Alexey Kryuchkov, some cosmetic adjustments by me
Report: https://postgr.es/m/153053285112.13258.434620894305716755@wrigleys.postgresql.org
Discussion: https://postgr.es/m/CAN85JcYphDLYt4CpMDLZjjNVqGDrFJ5eS3YF=wLAhFoDQuBsyg@mail.gmail.com
A critical failure in some of the end-of-recovery actions before the
end-of-recovery record is written can cause PostgreSQL to react
inconsistently with the rest of the cluster in the event of a crash
before the final record is written. Two such failures are for example
an error while processing a two-phase state files or when operating on
recovery.conf. With this commit, the failures are still considered
FATAL, but the write of the timeline history file is delayed as much as
possible so as the window between the moment the file is written and the
end-of-recovery record is generated gets minimized. This way, in the
event of a crash or a failure, the new timeline decided at promotion
will not seem taken by other nodes in the cluster. It is not really
possible to reduce to zero this window, hence one could still see
failures if a crash happens between the history file write and the
end-of-recovery record, so any future code should be careful when
adding new end-of-recovery actions. The original report from Magnus
Hagander mentioned a renamed recovery.conf as original end-of-recovery
failure which caused a timeline to be seen as taken but the subsequent
processing on the now-missing recovery.conf cause the startup process to
issue stop on FATAL, which at follow-up startup made the system
inconsistent because of on-disk changes which already happened.
Processing of two-phase state files still needs some work as corrupted
entries are simply ignored now. This is left as a future item and this
commit fixes the original complain.
Reported-by: Magnus Hagander
Author: Heikki Linnakangas
Reviewed-by: Alexander Korotkov, Michael Paquier, David Steele
Discussion: https://postgr.es/m/CABUevEz09XY2EevA2dLjPCY-C5UO4Hq=XxmXLmF6ipNFecbShQ@mail.gmail.com
If a view references a foreign table, and the foreign table has a
BEFORE INSERT trigger, then it's possible for a tuple inserted or
updated through the view to be changed such that it violates the
view's WITH CHECK OPTION constraint.
Before this commit, postgres_fdw handled this case inconsistently. A
RETURNING clause on the INSERT or UPDATE statement targeting the view
would cause the finally-inserted tuple to be read back, and the WITH
CHECK OPTION violation would throw an error. But without a RETURNING
clause, postgres_fdw would not read the final tuple back, and WITH
CHECK OPTION would not throw an error for the violation (or may throw
an error when there is no real violation). AFTER ROW triggers on the
foreign table had a similar effect as a RETURNING clause on the INSERT
or UPDATE statement.
To fix, this commit retrieves the attributes needed to enforce the
WITH CHECK OPTION constraint along with the attributes needed for the
RETURNING clause (if any) from the remote side. Thus, the WITH CHECK
OPTION constraint is always evaluated against the final tuple after
any triggers on the remote side.
This fix may be considered inconsistent with CHECK constraints
declared on foreign tables, which are not enforced locally at all
(because the constraint is on a remote object). The discussion
concluded that this difference is reasonable, because the WITH CHECK
OPTION is a constraint on the local view (not any remote object);
therefore it only makes sense to enforce its WITH CHECK OPTION
constraint locally.
Author: Etsuro Fujita
Reviewed-by: Arthur Zakirov, Stephen Frost
Discussion: https://www.postgresql.org/message-id/7eb58fab-fd3b-781b-ac33-f7cfec96021f%40lab.ntt.co.jp
Commit bc292937ae failed to update a comment about unique index
checking. _bt_insertonpg() is no longer responsible for finding an
insertion location while preventing conflicting insertions.