refactor(sql): promote the single-row limit onto ISqlDialect
Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
This commit is contained in:
Joseph Doherty
@@ -6,9 +6,9 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Sql;
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/// <summary>
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/// The provider seam (design §2.2). ADO.NET's <see cref="System.Data.Common"/> base types abstract
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/// connections, commands, parameters and readers; they do <b>not</b> abstract the two things that differ
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/// per backend — <b>identifier quoting</b> and the <b>metadata-catalog SQL</b>. Those live here, so no
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/// dialect assumption leaks into the reader, the poll planner, or the schema browser.
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/// connections, commands, parameters and readers; they do <b>not</b> abstract the three things that differ
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/// per backend — <b>identifier quoting</b>, the <b>metadata-catalog SQL</b>, and <b>row-limit syntax</b>.
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/// Those live here, so no dialect assumption leaks into the reader, the poll planner, or the schema browser.
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/// <para><b>This interface is the driver's SQL-injection boundary.</b> Every runtime <em>value</em> is
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/// bound as a <see cref="DbParameter"/> and never reaches a command text. Identifiers cannot be
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/// parameterized in SQL, so the few that must appear as text are sourced only from catalog-validated
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@@ -47,6 +47,41 @@ public interface ISqlDialect
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/// <summary>Cheapest statement that proves the connection is alive (<c>SELECT 1</c>; Oracle needs <c>FROM DUAL</c>).</summary>
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string LivenessSql { get; }
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/// <summary>
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/// The fragment that goes <b>immediately after <c>SELECT</c></b> to limit a statement to one row —
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/// T-SQL's <c>"TOP 1 "</c>. Empty for a dialect that spells the limit at the end of the statement (see
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/// <see cref="SingleRowLimitSuffix"/>).
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/// <para><b>Include the trailing space</b> when non-empty: the planner concatenates
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/// <c>"SELECT " + SingleRowLimitPrefix + columns</c> with no separator of its own, so an empty fragment
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/// must leave the statement byte-identical to an unlimited one.</para>
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/// </summary>
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/// <remarks>
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/// <para><b>Why a prefix/suffix pair rather than one <c>ApplyRowLimit(sql, n)</c> method.</b> The two
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/// spellings sit at opposite ends of the statement — SQL Server writes
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/// <c>SELECT TOP 1 … ORDER BY … DESC</c>, while SQLite/Postgres/MySQL write
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/// <c>SELECT … ORDER BY … DESC LIMIT 1</c>. A single method taking the SELECT list could only serve the
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/// prefix position; a single method taking the whole statement would move statement assembly out of the
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/// planner and into every dialect. Two fragments keep the planner the sole author of statement shape
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/// and let each dialect fill in the end it uses.</para>
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/// <para><b>Why "single row" rather than a row count.</b> The only limit v1 emits is the wide-row
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/// <c>topByTimestamp</c> selector's newest-row pick. Modelling an arbitrary <c>n</c> would be untested
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/// surface, and <c>FETCH FIRST</c>/<c>ROWNUM</c> dialects need more than a substituted number anyway.
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/// Widen this to a method when a feature actually needs <c>n > 1</c>.</para>
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/// </remarks>
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string SingleRowLimitPrefix { get; }
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/// <summary>
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/// The fragment appended to the <b>very end</b> of a statement to limit it to one row —
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/// <c>" LIMIT 1"</c> for SQLite/Postgres/MySQL. Empty for a dialect that limits after <c>SELECT</c>
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/// (see <see cref="SingleRowLimitPrefix"/>).
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/// <para><b>Include the leading space</b> when non-empty, for the same reason the prefix carries a
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/// trailing one: the planner appends it directly to the finished statement.</para>
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/// <para>Exactly one of the two fragments is non-empty in every dialect modelled so far, but the planner
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/// emits <b>both</b> unconditionally — a dialect needing both ends (or neither) is expressible without
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/// touching the call site.</para>
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/// </summary>
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string SingleRowLimitSuffix { get; }
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/// <summary>Catalog query listing schemas — the browser's <c>RootAsync</c> level. Takes no parameters.</summary>
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string ListSchemasSql { get; }
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@@ -41,8 +41,7 @@ public static class SqlGroupPlanner
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/// up front, so reaching one here means a caller built a definition by hand with a broken invariant.
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/// </exception>
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/// <exception cref="NotSupportedException">
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/// A tag carries <see cref="SqlTagModel.Query"/> (design §5.4, deferred to P3), or a
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/// <c>topByTimestamp</c> selector is planned for a provider whose row-limit syntax is not yet modelled.
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/// A tag carries <see cref="SqlTagModel.Query"/> (design §5.4, deferred to P3).
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/// <b>Deliberately loud rather than skipped:</b> silently dropping a tag would leave an authored node
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/// permanently unfed with nothing in the log, and the parser makes this branch unreachable in practice.
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/// </exception>
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@@ -157,7 +156,10 @@ public static class SqlGroupPlanner
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/// <summary>
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/// Emits <c>SELECT <cols> FROM <table> WHERE <whereColumn> = @w</c>, or — for a
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/// <c>topByTimestamp</c> selector — <c>SELECT TOP 1 <cols> FROM <table> ORDER BY <ts> DESC</c>.
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/// <c>topByTimestamp</c> selector — the single-row form
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/// <c>SELECT <limit> <cols> FROM <table> ORDER BY <ts> DESC <limit></c>, whose
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/// row limit comes from <see cref="ISqlDialect.SingleRowLimitPrefix"/> /
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/// <see cref="ISqlDialect.SingleRowLimitSuffix"/> (T-SQL fills the prefix: <c>SELECT TOP 1 …</c>).
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/// <para>The SELECT list is each member's <c>columnName</c> (distinct, in first-appearance order),
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/// followed by each distinct member <c>timestampColumn</c> — so members of one row may carry different
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/// timestamp columns without splitting the group. The where-column itself is <b>not</b> selected: every
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@@ -191,18 +193,13 @@ public static class SqlGroupPlanner
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if (!Blank(first.RowSelectorTopByTimestamp))
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{
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// "TOP 1" is T-SQL. Other providers spell the row limit differently (LIMIT 1, FETCH FIRST,
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// ROWNUM), and ISqlDialect does not model it yet because v1 constructs SqlServer only — so refuse
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// loudly rather than emit a statement that would not parse. Promote a row-limit member onto
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// ISqlDialect when P2 adds Postgres/ODBC.
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if (dialect.Provider != SqlProvider.SqlServer)
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throw new NotSupportedException(
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$"The wide-row 'topByTimestamp' row selector needs a provider-specific row limit; " +
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$"{dialect.Provider} is not modelled yet (v1 constructs SqlServer only).");
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// The row limit is dialect syntax and sits at opposite ends of the statement per provider
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// (T-SQL "SELECT TOP 1 …", SQLite/Postgres/MySQL "… LIMIT 1"), so both ends are emitted
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// unconditionally and the dialect decides which one it fills. See ISqlDialect.SingleRowLimitPrefix.
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var sql = string.Concat(
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"SELECT TOP 1 ", columns, " FROM ", table,
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" ORDER BY ", dialect.QuoteIdentifier(first.RowSelectorTopByTimestamp!), " DESC");
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"SELECT ", dialect.SingleRowLimitPrefix, columns, " FROM ", table,
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" ORDER BY ", dialect.QuoteIdentifier(first.RowSelectorTopByTimestamp!), " DESC",
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dialect.SingleRowLimitSuffix);
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return new SqlQueryPlan(SqlTagModel.WideRow, groupKey, sql, [], [], members, selected);
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}
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@@ -29,6 +29,16 @@ public sealed class SqlServerDialect : ISqlDialect
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/// <inheritdoc/>
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public string LivenessSql => "SELECT 1";
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/// <summary>
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/// T-SQL limits after the <c>SELECT</c> keyword, so the whole limit lives in the prefix —
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/// <c>"TOP 1 "</c>, trailing space included so <c>SELECT TOP 1 [col]</c> composes with no extra
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/// separator at the call site.
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/// </summary>
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public string SingleRowLimitPrefix => "TOP 1 ";
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/// <summary>Empty — T-SQL has no trailing row-limit clause (<c>OFFSET/FETCH</c> is a paging construct, not this).</summary>
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public string SingleRowLimitSuffix => string.Empty;
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/// <inheritdoc/>
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public string ListSchemasSql =>
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"SELECT DISTINCT TABLE_SCHEMA FROM INFORMATION_SCHEMA.TABLES ORDER BY TABLE_SCHEMA";
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