Resolve INSERT conflicts through the index instead of a prior lookup - #623
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Included review availability: This review used your included allowance. Your plan provides up to 4 included reviews per hour; 0 remain after this review. 📝 WalkthroughWalkthroughApplied inserts now use cached arbiter indexes and speculative retries to handle conflicts. A postmaster GUC enables direct inserts without conflict lookup. Compatibility macros and a TAP test cover the insert path across PostgreSQL versions and a controlled conflict race. The pgindent script also changes its awk extraction method. ChangesInsert Conflict Handling
pgindent Portability
Priority: ➖ Normal Merge Risk: 🟡 Moderate · up to A concurrent local insert can still cause replication of the conflicting row to fail instead of resolving the conflict. Address or explicitly accept this remaining race before merging. Security Architecture ReviewSecurity architecture risk: 🔵 Low · up to The normal path gains bounded conflict retries. The new bypass is off by default, but enabling it without a genuine guarantee that keys cannot collide can cause replicated transactions to be discarded or a subscription to be disabled. Retained concerns
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A rabbit checks the index rows, Comment |
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Inline comments:
In `@src/spock_apply_heap.c`:
- Around line 1154-1169: In the speculative insert path, update the code around
spock_prepare_insert_tuple() and spock_speculative_insert() to execute both
helpers under the relation owner context using SwitchToUntrustedUser() and
RestoreUserContext(). Preserve each helper’s result and existing control flow,
including breaking when preparation skips the row or speculative insertion
succeeds.
In `@src/spock_relcache.c`:
- Around line 238-239: Update spock_relation_open() to set entry->arbiterIndexes
to NIL immediately after freeing the existing list, before
SpockBuildInsertArbiterIndexes() rebuilds it; preserve the existing behavior
when the list is already NIL.
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Actionable comments posted: 1
- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@src/spock_apply_heap.c`:
- Around line 1158-1165: Update spock_apply_heap_insert() so an undiscoverable
duplicate after the speculative-insertion attempt limit uses an insert path that
preserves the tuple prepared by spock_prepare_insert_tuple() and allows the
unique index to raise ERRCODE_UNIQUE_VIOLATION. Avoid re-invoking
ExecSimpleRelationInsert() unless the preparation steps are intentionally
repeated, and leave apply_work() retry handling unchanged.
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| * The AM waits for a competing inserter before reporting, so a conflict |
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With noDupErr=true, PostgreSQL uses UNIQUE_CHECK_PARTIAL. If it sees a possible duplicate, it immediately reports a conflict. It does not wait to learn whether the other transaction commits or rolls back.
That means this comment is incorrect:
“the AM waits for a competing inserter before reporting”
The retry loop can therefore run all three attempts very quickly while the competing transaction is still open. After that, the code falls back to a normal insert. That insert does wait, but if the other transaction commits, it raises the same unhandled duplicate-key error this PR is trying to prevent.
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@rasifr ,
As I see it, we have two tasks:
- How to politely detect concurrent insertions and process them correctly.
- How to (and how long) wait for concurrent transaction(s) in anticipation that they commit and resolve the conflict in a natural way.
I propose to separate these tasks. Here we can solve speculative insertion, and in the next one, the waiting cycle. If you agree, I will wrap this task up with proper comments.
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Agreed, splitting makes sense. Please update the comments so this PR clearly reflects its current scope, then open a follow up PR for the waiting cycle.
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Agreed, splitting makes sense. Please update the comments so this PR clearly reflects its current scope
Thanks. I adjusted this PR a little.
then open a follow up PR for the waiting cycle.
Not so fast, we need to implement it in advance ;)
Collect a relation's immediate unique indexes when its mapping is built and keep the list beside the other state derived from that mapping. Nothing uses it yet. The apply path does in the next commit, where it has to name the indexes that may reject a tuple it is about to store, and recomputing that for every applied row would be wasted work on a hot path. Like the delta-apply metadata next to it, the list has to be rebuilt explicitly whenever the mapping is rebuilt: a relcache invalidation only resets reloid, so a stale list would otherwise outlive the index set it was derived from. The apply path already forces pending invalidations to be processed before it uses a cached relation, which is what makes rebuilding at mapping time sufficient. Deferrable unique indexes are left out. They are checked at commit rather than at insert, and the executor only reports an insert-time conflict for immediate ones, so listing them would achieve nothing.
spock_apply_heap_insert() decided whether an incoming row conflicted by probing for a local tuple and, finding none, falling straight into ExecSimpleRelationInsert(). A probe cannot settle that question. It runs a scan that a concurrent local writer can race, and it runs before the store in any case, so a row committed in between is missed either way. Acting on a stale "no conflict" turned an insert on a relation where the operator had configured conflict resolution into a duplicate key violation, left for the exception machinery to clean up. Store the tuple speculatively instead, with every immediate unique index as an arbiter. The index AM reports a duplicate back through specConflict rather than raising it; the speculative tuple is super-deleted, and the next pass looks the conflicting row up and hands it to the normal conflict resolution path. Losing the race costs a super-deleted tuple; winning it costs the speculative token and its confirm record. The AM does not wait before reporting. noDupErr makes the check UNIQUE_CHECK_PARTIAL, so a key held by a transaction still in progress is reported at once. The waiting happens in the lookup instead: its dirty-snapshot scan waits on an in-progress holder, so if the holder commits the next pass finds its row, and if it aborts the next store succeeds. The lookup still runs first. It settles the common case without storing anything, and its false negatives are now harmless -- the same bargain core strikes in check_exclusion_or_unique_constraint(). One loop now answers for both levels. The lookup settles the common case and supplies the row that conflict resolution needs; the index catches what the lookup missed and sends the loop round again. Nothing here makes an outcome worse than before. Whatever the loop cannot resolve ends where every such insert ended before this change: in a plain insert that waits for an in-progress holder and raises a duplicate key if it commits. The misses themselves -- the lookup racing a concurrent writer, and the dirty-snapshot scan skipping a row being updated -- are not new; what is new is that the index reports them instead of raising, and a second pass gets the chance to resolve them. The price is the speculative token and its confirm record on every insert into a relation with a unique index, and a super-deleted tuple per lost race. Arbitration covers every unique index while the lookup covers only those the operator opted into, so an arbiter can report a conflict the lookup is not allowed to chase: a clash on a secondary unique index, where the replica identity search looks for a different key and finds nothing. Treating the row found through some other index as the same row is the policy spock.check_all_uc_indexes gates, so we do not take it upon ourselves. After three passes the tuple is stored with the indexes free to raise, and the duplicate key, naming the constraint and the key values, goes to spock.exception_behaviour. In that case nothing waits for a holder that is still in progress: the passes are spent at once, and the final plain insert waits and then raises if the holder commits. How, and how long, to wait for a concurrent inserter is a separate problem, left to a follow-up. This commit only makes sure a concurrent insertion is detected and routed to conflict resolution wherever the lookup can see it. Relations with no immediate unique index keep the plain insert path: nothing there can reject the tuple, so there is nothing to arbitrate. This also answers the TODO asking whether the insert path needed the retry loop the UPDATE and DELETE paths have. It needs a different thing: an arbiter that sees the key under a page lock. ExecInsertIndexTuples() gained its last argument in PG16 and was reshaped in PG19, where the booleans became an EIIT_* bitmask and the parameters were reordered, so the call needs a compatibility macro on both ends.
The race -- a conflicting local row committed between the apply worker's lookup and its store -- cannot be hit reliably by timing, so hold the worker in exactly that window and commit the conflicting row while it waits. The index then reports the duplicate and the next pass resolves against the row that appeared. That window is Spock's own code, so the injection point can live here instead of in core, and the test needs no patched server: it skips unless the core injection_points module is installed, and the point compiles to nothing unless the server was built with --enable-injection-points. It follows the existing spock_injection.h pattern, including the argument count the macro takes on each supported major version. What this covers is our half of the problem: that the apply path survives a lookup answer that went stale, whatever made it stale. Reproducing the stale answer through the dirty-snapshot index scan that produces it in the field would mean pausing inside core's index_getnext_slot(), which an extension cannot reach; that belongs with the fix proposed upstream.
Applying an INSERT costs two pieces of work that only a possible collision with a local row justifies: the lookup for that row, and the speculative store that lets the unique indexes report a duplicate instead of raising one. Where the key ranges are partitioned between the nodes, no applied insert can collide and both are pure overhead. Add a boolean GUC with which the operator asserts exactly that. With it on the apply worker skips the lookup and the speculative store and inserts the row directly. Nothing verifies the assertion: a collision then raises a duplicate key error, which spock.exception_behaviour disposes of as it does any other apply error. Off by default. The test arms the stall injection point and checks that an insert replicates straight through it, which it can only do if neither path ran.
The scan used match(s, r, arr), a gawk extension that captures a group into an array. macOS ships the one true awk as /usr/bin/awk, where match() takes two arguments, so the program did not parse at all there and the script died before reaching pgindent. Peel the typedef name off the closing line with sub() instead, which every awk has. No change to what the scan recognises.
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Approved. Though has to wait for the other PR to solve waiting cycle.
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Note
This PR introduces no new failure mode. Both ways the lookup can miss a conflicting row that predate it; what it adds is handling one level lower. A single loop now covers both levels: the lookup settles the common case and supplies the row for conflict resolution, the index catches what the lookup missed and sends the loop round again. Anything the loop cannot resolve ends exactly as it did before - in a plain insert that waits for the holder and raises a duplicate key if it commits.
Problem
spock_apply_heap_insert() decided whether an incoming row conflicted with a local one by probing for it and, finding nothing, falling straight into ExecSimpleRelationInsert().
A probe cannot settle that question. It runs an index scan that a concurrent local writer can race:
Acting on a stale "no conflict" meant an insert on a relation where the operator had configured conflict resolution ended as duplicate key value violates unique constraint, left for spock.exception_behaviour to dispose of: a discarded transaction, or a disabled subscription.
Approach
Reuse standard speculative insertion. It adds a sensible overhead (~10-15% with the pgbench benchmark), so we add an option to avoid it - if the user guarantees it has a unique cross-node IDENTITY (like snowflake) and has no conflict at all.
The tuple is now stored speculatively, with every immediate unique index of the relation as an arbiter. ExecInsertIndexTuples() reports a duplicate through specConflict rather than raising it; the speculative tuple is super-deleted, the lookup on the next pass finds the row that appeared, and it goes through the normal conflict resolution path. Losing the race costs a super-deleted tuple; winning it costs the speculative token and its confirm record.
The lookup still runs first — it settles the common case without storing anything, and its false negatives are now harmless. That is the same bargain core strikes in check_exclusion_or_unique_constraint().