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Query

property Query: TvxClientDataSet read write

Example​

procedure ScriptEvent(var Value: variant);
var
  Rows: TvxClientDataSet;
begin
  Rows := Dataview.Query;
  if Rows.Active then
    Value := Rows.RecordCount
  else
    Value := 0;

  // Do not free Rows and do not assign Dataview.Query directly.
end;

Usage​

Query returns the SQL view's live client dataset. Direct replacement can permanently change the view and should not be used in normal scripts.

Behaviour

  • Reading returns the live SQL result dataset. Query, the view and the report pipeline share one cursor, so navigation through any of them changes the current record seen by the others.
  • Do not assign a different dataset in normal scripts. The first different assignment disposes of the existing query and permanently changes how the view handles later replacements and opening errors.
  • Reset and deactivation can close the dataset and invalidate saved fields or bookmarks even though the Query reference remains non-nil.

Errors

Dataset operations can raise for inactive state, missing fields, invalid cursor/edit state, SQL/connection failures and constraints. Unsafe replacement can cause nil dereferences, invalid object access, stale metadata, dangling references or delayed failures in activation/saving.

Usage notes

Treat direct assignment as an advanced operation, not a routine data-loading API. Use LinkDataView when a Velox helper requires a view for an existing dataset. Never retain Query beyond the dataset or view that provides it.

Additional Technical Info​

Query returns the live client dataset used by this SQL view, its DataSource and report pipeline. Normal scripts should treat the returned object as borrowed: use it within the current event/flow, do not free it and do not assign the property directly.

The example performs a read-only observation without retaining the reference. It is source-reviewed and was not executed by the documentation workflow.

The compiler-visible type is TvxClientDataSet. Native storage and runtime helpers use the more specialised TdaChildSQLClientDataSet, creating a wider script contract than the implementation actually expects.

Implementation

The constructor creates TdaChildSQLClientDataSet(Self), applies the owner definition's string-trimming setting and connects the owned DataSource. In the normal state, the view explicitly frees that dataset at destruction. View navigation, field access, link indexes, activation/reset and record/key operations use the FQuery pointer.

The setter supports Velox's internal LinkDataView(aDataSet) adapter. On the first different value it frees the normal owned Query and permanently sets FQueryAssigned. It then stores the supplied pointer and reconnects FDataSource.DataSet. Subsequent different values replace only that borrowed pointer. When FQueryAssigned is True, the view destructor does not free the current Query.

LinkDataView creates/reuses a temporary SQL-view wrapper, casts the supplied TvxClientDataSet to the child SQL class and assigns this property. Scripts should call that helper rather than reproduce the setter protocol.

Edge cases and quirks

  • Passing nil destroys the original dataset, sets DataSource.DataSet to nil and leaves the irreversible assigned state. Later view operations dereference nil.
  • The compiler accepts any TvxClientDataSet, while the runtime helper performs no explicit specialisation check. SQL-child-only operations may touch incompatible object state.
  • Replacement does not apply DisableStringTrim, SQL connection, SQL text, packet/link/index settings or other normal child initialisation to the supplied dataset.
  • The view no longer owns an assigned dataset. If its real owner frees it first, Query and DataSource become dangling; if nobody owns/frees it, it leaks.
  • Reassigning cannot restore the constructor-owned lifecycle because FQueryAssigned never becomes False and the original object has already been freed.
  • The DataSource is reconnected, unlike TdaCDSQueryDataView.Query, but dependent view SQL/key metadata can still describe a different dataset.

Side effects

Reading has no immediate side effect. First different assignment frees an object and changes ownership mode; every different assignment changes the shared dataset behind the DataSource/pipeline.

Performance and concurrency

The getter and pointer switch are constant-time, but first replacement destroys the existing dataset and its buffers. Dataset operations scale with records, fields, indexes and BLOBs. There is no synchronisation across Query, view, DataSource and pipeline.

External references

Created 2026-07-15