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| Check name | Status | Explanation | Resolution |
|---|---|---|---|
| Title check |  |
The title correctly mentions per-row hashing and combining row hashes, but it incorrectly states that the implementation uses XOR. The implementation hashes the row count and four signed 64-bit sums, … | Update the title to describe the implemented block-hash algorithm, such as "Hash each row and combine row-hash sums". |
| Docstring Coverage | |
Docstring coverage is 60.61% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 33 functions across 11 files. (2 skipped:… | Write docstrings for the functions missing them to satisfy the coverage threshold. |
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Check name Status Explanation Description check The description directly explains the hashing problem, row-hash changes, block-hash aggregation, hash-version handling, numeric arrays, and pgcrypto removal. It is related to the changeset. Linked Issues check Check skipped because no linked issues were found for this pull request. Out of Scope Changes check Check skipped because no linked issues were found for this pull request.
Full details: Title check
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Explanation
The title correctly mentions per-row hashing and combining row hashes, but it incorrectly states that the implementation uses XOR. The implementation hashes the row count and four signed 64-bit sums, so the title is misleading about the primary algorithm.
Full details: Docstring Coverage
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Docstring coverage is 60.61% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 33 functions across 11 files. (2 skipped: 2 unsupported.)
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I’m a rabbit counting hashes in the moonlight,
Four word-sums hop beside each row,
Old leaf marks get a fresh spring coat,
Nulls and empty strings keep their place,
Pgcrypto rests beneath the clover,
And version four bounds away.
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Up to standards ✅🟢 Issues
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| Category | Results |
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| Complexity | 2 medium |
🟢 Metrics -2 complexity · -3 duplication
Metric Results Complexity -2 Duplication -3
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Review comments at @db/queries/templates.go:
- Around line 1419-1434: Update TDBlockHashSQL and MtreeLeafHashSQL to include
the row count alongside the XOR result when computing each aggregate hash.
Ensure both hashes distinguish duplicate rows from an empty block while
preserving their existing output formats.
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📒 Files selected for processing (11)
db/queries/queries.godb/queries/queries_test.godb/queries/templates.godocs/CHANGELOG.mdinternal/cli/cli.gointernal/consistency/diff/table_diff.gointernal/consistency/mtree/merkle.gointernal/consistency/mtree/merkle_test.gotests/integration/block_hash_test.gotests/integration/main_test.gotests/integration/native_pg_test.go
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- tests/integration/native_pg_test.go
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| // 1: string_agg of whole-row ::text, hashed with pgcrypto digest(). | ||
| // 2: string_agg of per-column concat_ws with trim_scale. | ||
| // 3: multiset hash of the sha256 hashes of the rows (see BlockHashAggExpr). | ||
| const CurrentHashVersion = 3 |
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Allocate a distinct hash version from current main
Merkle trees persist in the database across ACE invocations. Current main (86af353f) already assigns version 3 to its hash-affecting session/output-setting change, while this PR uses version 3 for a different block-hash algorithm.
If any tree is built with current main before this change is deployed, the new binary will see version 3 and skip the full recomputation. Clean leaves will retain the earlier algorithm, while subsequently dirtied leaves can be recomputed with this PR’s algorithm.
Please rebase onto current main and assign version 4 to the multiset-hash change. If it is guaranteed that current-main v3 can never be run or distributed independently, please document that release constraint explicitly.
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Hmm, all these PRs by-design should be applied in the same release. If something changes, we can bump the version again.
Anyway, applying different values in these PRs is risky, what if a commit with hash version 4 is pushed before 3? So, I used the same value just to get a conflict and fix it manually at the pre-merge stage.
| return nil, fmt.Errorf("row hash has %d bytes, want %d", len(h), len(entry.hash)) | ||
| } | ||
| copy(entry.hash[:], h) | ||
| result[key] = entry |
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Preserve duplicate-key multiplicity after detecting the mismatched leaf.
The new block hash correctly distinguishes duplicate rows, but this map still retains only one entry per primary-key value. In an inheritance tree, a parent and child can contain rows with the same key, so the later row silently overwrites the earlier one here.
For example, {X, X} on one node versus {X} on another produces different leaf hashes, but both row-hash maps become key -> X. The row-level comparison then reports no difference, leaving the leaf inconsistent without an actionable row to report or repair. common.rowsToMap has the same multiplicity loss in table-diff.
Please retain all entries for a key—or explicitly reject this table shape. If repair is intended, the representation will also need enough physical relation identity to distinguish the parent copy from the child copy. An end-to-end {X, X} versus {X} test would expose this; the current test covers {X, X} versus no X, which still reports one row after the overwrite.
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@rasifr ,
The analysis is correct.
I would like to keep this out of the current PR, for two reasons:
- The problem is older than this PR. The previous block hash (string_agg over all rows of the block) also saw both copies of the row, and the same two maps also kept only one of them. So the state "the block differs, but no row differs" was already possible with the old code. This PR does not add it. It only makes the block hash count each copy, which is required for detection anyway.
- A correct fix is a separate piece of work. The comparison must keep all rows for a key, the report must show the extra copies, and repair needs the physical relation of each copy (for example, tableoid). This changes the row representation in the diff output and in repair, not only the hash.
The scope of this PR is the out-of-memory failure on large blocks and the row encoding collisions (NULL against an empty string, moved delimiters).
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Please add a compatibility guard for newer stored hash versions The migration path only handles It can then recompute dirty leaves with its older algorithm while Please fail explicitly when |
Done |
isNumericType and isNumericColType matched every type name that starts with "numeric" or "decimal", so a numeric[] column was wrapped in trim_scale() too. trim_scale has no array variant, and the block hash and row hash queries failed on every table with such a column. Arrays are no longer matched.
The hash of an mtree leaf or a table-diff block joined the text of all
its rows into one string with string_agg and then hashed it. On a block
with a few million rows this string grew past the 1 GB limit on one
value, and the build failed with "out of memory" (SQLSTATE 54000).
A customer hit this on a table of 3.79 billion rows: the sampled block
ranges gave some blocks millions of rows. A larger work_mem does not
help, because the limit is fixed.
The text of a row also lost information. concat_ws('|', ...) with
COALESCE(col::text, '') gave the same text for a NULL and for an empty
string, and for the rows ('a|b', 'c') and ('a', 'b|c'). The diff did not
see such differences.
Now each row is hashed separately. The row hash is
sha256(convert_to(ROW(...)::text, 'UTF8')). The record output of ROW()
keeps NULL and empty strings apart and quotes delimiters, and it has no
limit of 100 arguments, so concatWSBatched is gone. Numeric columns
still go through trim_scale. convert_to() gives the same bytes on nodes
with different database encodings, which logical replication allows,
and sha256 does not depend on the byte order of the server.
The block hash is the XOR of the row hashes: each hash is cast through
hex to bit(256), bit_xor combines them, and bit_send gives the 32 bytes
back. The aggregate state is one bit(256), whatever the number and the
width of the rows. The order of rows does not matter: the primary key is
part of every row hash, and a key appears at most once in a block.
The row hashes of the mtree diff use the same expression
(queries.RowHashExpr). Otherwise a leaf could differ while every row
hash matches, and the diff would not fetch the rows of that block. The
mtree diff reads the row hash as 32 raw bytes, not as text.
RowHashExpr returns an error for an empty column list: the whole-row
value of the table would be another encoding.
bit_xor needs PostgreSQL 14 or later. CurrentHashVersion is now 4, so
the next mtree update computes all stored leaf hashes again.
TestBlockHashEncoding loads rows into temporary tables on one node and compares the mtree leaf hash and the table-diff block hash of each table. The two hashes must always be the same. Different rows must give different hashes: a NULL and an empty string, a delimiter that moves from one column to the next, two rows that join into one row of the same text, rows that swap their values, and an extra row. The last two cases matter because bit_xor does not depend on the order of rows. Equal rows must give equal hashes, also when they were inserted in another order or when numeric values differ only in trailing zeros. The test also checks the hash of an empty block and a table with 150 columns. For every table, the leaf must be the XOR of the row hashes that the mtree diff reads, and a block of one row must hash to the sha256 of its row text. This checks the whole formula on the server, including the way the bit(256) value becomes 32 bytes again. The same rows must also give the same hash in a LATIN1 database and in a UTF8 database. The test skips this case if it cannot create a LATIN1 database. With the earlier hash, NULL against an empty string, the moved delimiter and the joined rows give equal hashes.
The block hash uses the built-in sha256(), so ACE no longer needs pgcrypto. mtree no longer creates the extension in RunChecks, and table-diff no longer creates it for --ensure-pgcrypto or suggests it when a hash query fails. The integration tests no longer create it, so they check that ACE works without it. --ensure-pgcrypto is kept as a hidden flag that only prints a warning, so that scripts that still pass it do not fail.
The block hash was the XOR of the row hashes. Two equal rows cancel each
other in a XOR. In an inheritance tree this can happen: the primary key
of the parent does not stop a child from having a row with the same key.
A block with the same row in the parent and in a child then had the same
hash as a block without this row, and table-diff did not look into it.
Now the row hash is cut into four 64-bit words, read as signed integers.
The block hash is the sha256 of the row count and the four sums of these
words. sum(bigint) gives a numeric, so the sums do not overflow. This is
a multiset hash: the order of rows still does not matter, and each copy
of a row counts. A row count added to the XOR is not enough: {X, X, Y}
and {Y, W, W} have the same count and the same XOR.
The inner query computes the row hash once, as bit(256). OFFSET 0 keeps
the planner from pulling it up into the aggregate, where it would be
computed once for each word. An empty block still hashes to 32 zero
bytes, so it does not change its parent node in the tree.
The block hash no longer uses bit_xor, so it does not need PostgreSQL 14.
TestBlockHashEncoding checks the formula against a Go version of it and
covers duplicate keys in a parent and its child. TestNativePG runs
table-diff on such a tree on two nodes.
All leaves of a tree must be computed by the algorithm that the stored hash version names. Two paths could break this. mtree update upgraded a tree whose version was older than CurrentHashVersion, but it accepted a newer version without a check. An older ACE that ran after a newer one could recompute the dirty leaves with its own algorithm and keep the newer version number. Nothing would then make the newer ACE recompute them. Now mtree update, and mtree table-diff through it, stop with an error if the stored version on any node is newer. The check runs before the CDC drain and before any node is changed, and again in the transaction that writes the leaves, because a newer ACE can rebuild the tree in between. refreshStaleLeaves rewrites some leaves and keeps the version. On a tree with an older version it would write leaves of the current algorithm under the older number. Now it requires the current version. Only mtree update may write into an older tree: it marks every leaf dirty, recomputes all of them and writes the current version in one transaction. mtree update also did not write the new version when no dirty block was left after the splits and the merges. A leaf stops being dirty only when its hash is computed again, so after an upgrade this case has no leaf of the older algorithm, and the version can be written. Every way out of the per-node work now goes through one function that writes the version after an upgrade and then commits. It also returns commit errors, which two early exits ignored. mtree build is still allowed on a newer tree: it writes every leaf and the version again, so it is the way back to an older ACE. ACE versions released before this one do not have the check. The CHANGELOG and the mtree update page say to rebuild the tree with mtree build of such a version before using it. TestMtreeRefusesNewerHashVersion checks that update and diff refuse a newer tree and change nothing, that build restores it, and that update upgrades an older tree. TestHashVersionError covers every case of the check.
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Problem
The hash of an mtree leaf or a table-diff block joined the text of all rows of the block into one string with
string_aggand hashed this string. On a block with a few million rows the string grew past the 1 GB limit on one value, and the query failed without of memory(SQLSTATE 54000).The row text also lost information.
concat_ws('|', COALESCE(col::text, ''), ...)gives the same text for a NULL and for an empty string, and for the rows('a|b', 'c')and('a', 'b|c'). The diff didn't catch these differences.Changes
sha256(convert_to(ROW(col1, col2, ...)::text, 'UTF8')). The output ofROW()keeps NULL and empty strings apart and quotes delimiters. Numeric columns still go throughtrim_scale().convert_to()produces the same bytes on nodes with different database encodings, which logical replication allows, and sha256 doesn't depend on the server's byte order.queries.RowHashExpr). Otherwise, a leaf could differ while every row hash matches, and the diff would not fetch that block's rows. The row hash is read as 32 raw bytes, not as text.CurrentHashVersionis now 3. The nextmtree updatecomputes all stored leaf hashes again, in one transaction per node.numeric[]column intrim_scale(), which has no array variant, so the hash queries failed on every table with such a column. Arrays are no longer wrapped.sha256()(PostgreSQL 11 and later). pgcrypto came into ACE together with the switch frommd5to sha256 for Merkle tree leaves (Use Merkle trees in ACE cli#253) and was used only fordigest(..., 'sha256'). ACE no longer creates the extension.table-diff --ensure-pgcryptoremains a hidden flag that only prints a warning, so scripts that still pass it do not fail.Requirement:
bit_xorneeds PostgreSQL 14 or later.