Write-skew reader query checks vertices across logical pairs #253

Problem

In acid.tex, the Write Skew test defines pair k as:

• p1.id = 2*k+1
• p2.id = 2*k+2

Therefore the logical pairs are (1,2), (3,4), ..., (19,20).

However, the WS T_R query in Listing fig:ws2 matches every pair of consecutive IDs:

MATCH (p1:Person),
      (p2:Person {id: p1.id+1})
WHERE p1.value + p2.value <= 0

This also checks cross-pair neighbors such as (2,3), even though those vertices belong to different logical pairs. Such a pair is not covered by the invariant and can produce a false write-skew violation.

The same query was propagated to several implementations in ldbc/ldbc_acid; see ldbc/ldbc_acid#15.

Suggested fix

Restrict p1 to the first (odd-ID) vertex of each logical pair:

MATCH (p1:Person),
      (p2:Person {id: p1.id+1})
WHERE p1.id % 2 = 1
  AND p1.value + p2.value <= 0

This makes the anomaly-check query consistent with the pair definition in the preceding test description.