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Compile Cypher to SQLite's SQL, reads and writes

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一周以上
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25/100
Issue 类型
功能
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活跃度
活跃
技术栈
python, sqlite
领域
backend, databases

调研方向

Start with the shared SQL compiler, the SQLite store from #80, and the shared spec for queries, writes, temporal behavior, and refusals. Then review the SQLite cross-engine contracts, noting which tests need a second writer and cannot run under SQLite's write lock. Done means the Cypher compiler, registered SQLite functions, serialized write steps, and stated test contracts work together.

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描述

Port of liberation-data/drivine4j203: the Cypher compiler's SQLite target, so a Cypher statement runs on a SQLite graph and gives the rows a graph engine gives. It builds on the SQLite store (#80).

What drivine4j does

  • A third dialect of the shared SQL compiler, beside Postgres and Oracle. A compiled statement opens with /* drivine:json */, so its text values are read as JSON.
  • Values are JSON text. A value that passes through a subquery is text again, so it is wrapped in json(…) wherever it goes into json_array, json_object, json_group_array or json_set.
  • Functions registered on each connection do what SQLite's JSON functions don't:
    • drivine_equal(a, b): Cypher equality. 1 equals 1.0, maps compare in any key order, null gives null.
    • drivine_contains(props, map): a pattern's map of properties.
    • drivine_add(a, b): + of numbers, lists (a null is taken as an item) and texts.
    • drivine_canonical(json): maps' keys in order, so DISTINCT and grouping treat two maps of the same entries as one.
    • drivine_upper, drivine_split, drivine_reverse, drivine_uuid.
    • drivine_regex_group, drivine_regexp_replace (groups written \1).
    • drivine_trunc: whole where it fits. A real within 2 ulps of a whole number counts as that number.
    • drivine_to_char: the to_char patterns the temporal SQL uses.
    • drivine_temporal(op, …): local, utc, extract, add_months, add_time, months_between, seconds_between, truncate.
  • Timestamps are fixed-width ISO text, YYYY-MM-DDTHH:MM:SS.ffffff. The statement's clock is a literal taken as it is compiled, so one statement reads one clock.
  • No LATERAL. An OPTIONAL MATCH, a CALL { … }, a walk or a shortest path gathers its rows as a JSON list of maps in a subquery: json_each((SELECT json_group_array(json_object(…)) FROM (…) q)). That subquery can read the outer row. Each column is read back with -> (JSON, NULLIF(…, 'null')) or ->>.
  • Id lists are ,1,2, text, as on Oracle.
  • An order puts nulls NULLS LAST ascending and NULLS FIRST descending. It sorts by x ->> '$'.
  • Writes are a procedure of steps: select-into, let, loop over a query's rows, if, raise, execute (RETURNING … INTO), return. The steps are serialised as JSON. Each variable a step's SQL reads becomes a numbered parameter (?1); literals and quoted names are skipped. drivine_cypher(steps, params) carries the steps out on the statement's connection under a SAVEPOINT.
  • =~ is text REGEXP ('^(?:' || pattern || ')\z').

Tests

  • Run the shared spec on SQLite: queries, writes, temporal, refusals.
  • Run the cross-engine contracts that seed in Cypher.
  • Contract tests that need a second writer inside an open transaction can't run on SQLite: a transaction holds the write lock from BEGIN IMMEDIATE.
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