leonardBang commented on code in PR #4540:
URL: https://github.com/apache/flink-cdc/pull/4540#discussion_r4091160154


##########
flink-cdc-runtime/src/main/java/org/apache/flink/cdc/runtime/operators/schema/common/ExistingTableSchemaExpander.java:
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@@ -0,0 +1,898 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.cdc.runtime.operators.schema.common;
+
+import org.apache.flink.cdc.common.annotation.Internal;
+import org.apache.flink.cdc.common.event.AddColumnEvent;
+import org.apache.flink.cdc.common.event.AlterColumnTypeEvent;
+import org.apache.flink.cdc.common.event.CreateTableEvent;
+import org.apache.flink.cdc.common.event.SchemaChangeEvent;
+import org.apache.flink.cdc.common.event.SchemaChangeEventType;
+import org.apache.flink.cdc.common.event.TableId;
+import org.apache.flink.cdc.common.exceptions.SchemaEvolveException;
+import org.apache.flink.cdc.common.pipeline.ExistingTableSchemaExpansionMode;
+import org.apache.flink.cdc.common.pipeline.SchemaChangeBehavior;
+import org.apache.flink.cdc.common.schema.Column;
+import org.apache.flink.cdc.common.schema.Schema;
+import org.apache.flink.cdc.common.sink.ExistingTableSchemaExpansionSupport;
+import org.apache.flink.cdc.common.sink.MetadataApplier;
+import org.apache.flink.cdc.common.types.BinaryType;
+import org.apache.flink.cdc.common.types.CharType;
+import org.apache.flink.cdc.common.types.DataType;
+import org.apache.flink.cdc.common.types.DataTypeFamily;
+import org.apache.flink.cdc.common.types.DataTypeRoot;
+import org.apache.flink.cdc.common.types.DecimalType;
+import org.apache.flink.cdc.common.types.LocalZonedTimestampType;
+import org.apache.flink.cdc.common.types.TimeType;
+import org.apache.flink.cdc.common.types.TimestampType;
+import org.apache.flink.cdc.common.types.VarBinaryType;
+import org.apache.flink.cdc.common.types.VarCharType;
+import org.apache.flink.cdc.common.types.ZonedTimestampType;
+import org.apache.flink.util.FlinkRuntimeException;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+/**
+ * Handles the initial {@link CreateTableEvent} for an existing target table 
according to the
+ * configured {@link ExistingTableSchemaExpansionMode}.
+ */
+@Internal
+public class ExistingTableSchemaExpander {
+
+    private static final Logger LOG = 
LoggerFactory.getLogger(ExistingTableSchemaExpander.class);
+
+    private final MetadataApplier metadataApplier;
+    private final ExistingTableSchemaExpansionSupport expansionSupport;
+    private final SchemaChangeBehavior schemaChangeBehavior;
+    private final ExistingTableSchemaExpansionMode mode;
+
+    public ExistingTableSchemaExpander(
+            MetadataApplier metadataApplier,
+            ExistingTableSchemaExpansionSupport expansionSupport,
+            SchemaChangeBehavior schemaChangeBehavior) {
+        this(
+                metadataApplier,
+                expansionSupport,
+                schemaChangeBehavior,
+                ExistingTableSchemaExpansionMode.TRY_EXPAND);
+    }
+
+    public ExistingTableSchemaExpander(
+            MetadataApplier metadataApplier,
+            ExistingTableSchemaExpansionSupport expansionSupport,
+            SchemaChangeBehavior schemaChangeBehavior,
+            ExistingTableSchemaExpansionMode mode) {
+        this.metadataApplier = metadataApplier;
+        this.expansionSupport = expansionSupport;
+        this.schemaChangeBehavior = schemaChangeBehavior;
+        this.mode = mode;
+    }
+
+    /**
+     * Handles the initial {@link CreateTableEvent} for an existing target 
table.
+     *
+     * <p>This is a two-phase contract: the expander first tries to 
check/expand the existing target
+     * table, then signals whether the sink should still apply the original 
{@link
+     * CreateTableEvent}.
+     *
+     * @return {@code true} if the caller should proceed to apply the original 
{@link
+     *     CreateTableEvent} to the sink; {@code false} means the framework 
has fully handled this
+     *     event and the original {@link CreateTableEvent} must not be 
applied. {@code CHECK} mode
+     *     returns {@code false} after a successful check so that no external 
DDL is issued by the
+     *     pipeline.
+     */
+    public boolean handleExistingTableCreation(CreateTableEvent 
createTableEvent) {
+        if (mode == ExistingTableSchemaExpansionMode.CHECK) {
+            // CHECK guards the initial table state and runs regardless of 
schema.change.behavior.
+            checkCompatibility(createTableEvent);
+            return false;
+        }
+        if (schemaChangeBehavior == SchemaChangeBehavior.IGNORE
+                || schemaChangeBehavior == SchemaChangeBehavior.EXCEPTION) {
+            // Keep the original rule: TRY_EXPAND/EXPAND skip framework-side 
handling here.
+            return true;
+        }
+        switch (mode) {
+            case TRY_EXPAND:
+                tryExpand(createTableEvent);
+                return true;
+            case EXPAND:
+                expandStrictly(createTableEvent);
+                return true;
+            case DISABLED:
+            default:
+                return true;
+        }
+    }
+
+    private void checkCompatibility(CreateTableEvent createTableEvent) {
+        Optional<Schema> targetSchema = 
queryTargetSchema(createTableEvent.tableId());
+        if (!targetSchema.isPresent()) {
+            throw new SchemaEvolveException(
+                    createTableEvent,
+                    String.format(
+                            "Existing target table %s does not exist. CHECK 
mode never creates tables; create the target table externally first.",
+                            createTableEvent.tableId()));
+        }
+        ExpansionPlan plan = analyze(createTableEvent, targetSchema.get());
+        // CHECK never issues DDL, so every difference - including missing 
columns and narrow
+        // column types that EXPAND could repair - makes the target table 
unable to contain the
+        // upstream schema.
+        if (!plan.incompatibilities.isEmpty()
+                || !plan.columnsToAdd.isEmpty()
+                || !plan.columnsToWiden.isEmpty()) {
+            throw incompatibleException(createTableEvent, plan);
+        }
+        LOG.info(
+                "Existing target table {} passed the schema compatibility 
check.",
+                createTableEvent.tableId());
+    }
+
+    private void tryExpand(CreateTableEvent createTableEvent) {
+        // Phase 1 - probe the target table. Failures here are "cannot expand 
safely" cases: the
+        // connector lacks DDL capability, or the target table does not exist 
yet. Delegating to the
+        // sink's original behavior is safe because no derived DDL has been 
issued.
+        if (!supportsAnyExpansionDdl()) {
+            LOG.info(
+                    "Neither ADD_COLUMN nor ALTER_COLUMN_TYPE is enabled or 
supported for target table {}. Delegating schema handling to the sink.",
+                    createTableEvent.tableId());
+            return;
+        }
+        Optional<Schema> targetSchema;
+        try {
+            targetSchema = queryTargetSchema(createTableEvent.tableId());
+        } catch (Exception e) {
+            LOG.warn(
+                    "Failed to read the existing target table {} before 
expansion. Delegating schema handling to the sink.",
+                    createTableEvent.tableId(),
+                    e);
+            return;
+        }
+        if (!targetSchema.isPresent()) {
+            LOG.info(
+                    "Target table {} does not exist. Delegating table creation 
to the sink.",
+                    createTableEvent.tableId());
+            return;
+        }
+
+        ExpansionPlan plan = analyze(createTableEvent, targetSchema.get());
+        for (String incompatibility : plan.incompatibilities) {
+            LOG.warn(
+                    "Target table {} has an unsupported difference: {}. 
Delegating it to the sink.",
+                    createTableEvent.tableId(),
+                    incompatibility);
+        }
+
+        // Keys are not repairable by expansion, so issuing the remaining 
column-level DDL would
+        // mutate a target table that can never serve the pipeline. Delegate 
the whole event to the
+        // sink instead, which also lets the connector's own key validation 
have the last word.
+        if (plan.keysIncompatible) {
+            LOG.warn(
+                    "Target table {} has keys differing from the pipeline 
schema, which expansion cannot realign. Skipping expansion and delegating 
schema handling to the sink.",
+                    createTableEvent.tableId());
+            return;
+        }
+
+        // The plan needs repair DDL, but the sink cannot perform the specific 
required event type.
+        // This is a capability gap, not a transient failure, so delegate to 
the sink instead of
+        // failing the job (mirrors applyPlan's capability checks, which throw 
for EXPAND mode).
+        if (!plan.columnsToAdd.isEmpty()
+                && 
!supportsSchemaEvolutionType(SchemaChangeEventType.ADD_COLUMN)) {
+            LOG.warn(
+                    "Target table {} is missing columns {}, but ADD_COLUMN is 
not enabled or supported by the sink. Delegating schema handling to the sink.",
+                    createTableEvent.tableId(),
+                    getColumnNames(plan.columnsToAdd));
+            return;
+        }
+        if (!plan.columnsToWiden.isEmpty()
+                && 
!supportsSchemaEvolutionType(SchemaChangeEventType.ALTER_COLUMN_TYPE)) {
+            LOG.warn(
+                    "Target table {} has narrow columns {}, but 
ALTER_COLUMN_TYPE is not enabled or supported by the sink. Delegating schema 
handling to the sink.",
+                    createTableEvent.tableId(),
+                    plan.columnsToWiden.keySet());
+            return;
+        }
+
+        // Phase 2 - apply and verify. At this point the expander has 
identified supportable
+        // differences and is about to issue derived DDL. A failure here 
(transient network/catalog
+        // error, DDL execution failure, or read-back verification failure) 
must propagate so the
+        // job fails over and the idempotent expander retries; otherwise the 
original
+        // CreateTableEvent
+        // would be applied to an existing table (typically a no-op) and the 
missing columns would
+        // be
+        // silently dropped forever.
+        applyPlan(createTableEvent, plan);
+        verifyExpansion(createTableEvent, plan, false);
+    }
+
+    private void expandStrictly(CreateTableEvent createTableEvent) {
+        Optional<Schema> targetSchema = 
queryTargetSchema(createTableEvent.tableId());
+        if (!targetSchema.isPresent()) {
+            LOG.info(
+                    "Target table {} does not exist. Delegating table creation 
to the sink.",
+                    createTableEvent.tableId());
+            return;
+        }
+        ExpansionPlan plan = analyze(createTableEvent, targetSchema.get());
+        if (!plan.incompatibilities.isEmpty()) {
+            throw incompatibleException(createTableEvent, plan);
+        }
+        // A fully compatible target table needs no DDL, so a missing DDL 
capability is only an
+        // error when differences actually require repair; applyPlan enforces 
that per event type.
+        applyPlan(createTableEvent, plan);
+        verifyExpansion(createTableEvent, plan, true);
+    }
+
+    private boolean supportsAnyExpansionDdl() {
+        return supportsSchemaEvolutionType(SchemaChangeEventType.ADD_COLUMN)
+                || 
supportsSchemaEvolutionType(SchemaChangeEventType.ALTER_COLUMN_TYPE);
+    }
+
+    /** Renders every unresolved difference of a plan as a bullet list. */
+    private static String describeDifferences(ExpansionPlan plan) {
+        StringBuilder differences = new StringBuilder();
+        for (String incompatibility : plan.incompatibilities) {
+            differences.append("\n - ").append(incompatibility);
+        }
+        for (Column columnToAdd : plan.columnsToAdd) {
+            differences
+                    .append("\n - target table is missing column \"")
+                    .append(columnToAdd.getName())
+                    .append("\"");
+        }
+        for (Map.Entry<String, DataType> columnToWiden : 
plan.columnsToWiden.entrySet()) {
+            differences
+                    .append("\n - target column \"")
+                    .append(columnToWiden.getKey())
+                    .append("\" is narrower than pipeline type ")
+                    .append(columnToWiden.getValue());
+        }
+        return differences.toString();
+    }
+
+    private SchemaEvolveException incompatibleException(
+            CreateTableEvent createTableEvent, ExpansionPlan plan) {
+        String message =
+                String.format(
+                        "Existing target table %s cannot contain the pipeline 
schema:%s",
+                        createTableEvent.tableId(), describeDifferences(plan));
+        String repairSuggestions = renderRepairSuggestions(createTableEvent, 
plan);
+        if (!repairSuggestions.isEmpty()) {
+            message +=
+                    String.format(
+                            "\nSuggested repair SQL templates (review and 
adjust to the target connector's DDL dialect before execution):%s",
+                            repairSuggestions);
+        }
+        return new SchemaEvolveException(createTableEvent, message);
+    }
+
+    /**
+     * Renders lightweight, review-oriented ALTER TABLE suggestions for the 
safely repairable
+     * differences. Differences that cannot be fixed safely never get a 
suggested statement.
+     */
+    private String renderRepairSuggestions(CreateTableEvent createTableEvent, 
ExpansionPlan plan) {
+        StringBuilder suggestions = new StringBuilder();
+        for (Column columnToAdd : plan.columnsToAdd) {
+            suggestions
+                    .append("\n - ALTER TABLE ")
+                    .append(createTableEvent.tableId())
+                    .append(" ADD COLUMN ")
+                    .append(columnToAdd.getName())
+                    .append(" ")
+                    .append(columnToAdd.getType())
+                    .append(";");
+        }
+        for (Map.Entry<String, DataType> columnToWiden : 
plan.columnsToWiden.entrySet()) {
+            suggestions
+                    .append("\n - ALTER TABLE ")
+                    .append(createTableEvent.tableId())
+                    .append(" ALTER COLUMN ")
+                    .append(columnToWiden.getKey())
+                    .append(" TYPE ")
+                    .append(columnToWiden.getValue())
+                    .append(";");
+        }
+        return suggestions.toString();
+    }
+
+    /**
+     * Analyzes the pipeline schema against the current target schema and 
derives the safe DDL plan.
+     * Differences that cannot be fixed by safe DDL are collected in {@link
+     * ExpansionPlan#incompatibilities}.
+     */
+    private ExpansionPlan analyze(CreateTableEvent createTableEvent, Schema 
currentTargetSchema) {
+        ExpansionPlan plan = new ExpansionPlan();
+        Schema pipelineSchema = createTableEvent.getSchema();
+        boolean columnNameCaseSensitive = 
expansionSupport.isColumnNameCaseSensitive();
+        ColumnIndex targetColumns = indexColumns(currentTargetSchema, 
columnNameCaseSensitive);
+        plan.targetColumns = targetColumns;
+        Set<String> ambiguousColumnNames =
+                new HashSet<>(
+                        indexColumns(pipelineSchema, columnNameCaseSensitive)
+                                .getAmbiguousColumnNames());
+        ambiguousColumnNames.addAll(targetColumns.getAmbiguousColumnNames());
+        Set<String> keyColumns =
+                getKeyColumns(pipelineSchema, currentTargetSchema, 
columnNameCaseSensitive);
+        // Table keys are never realignable by ADD_COLUMN or 
ALTER_COLUMN_TYPE, and a target table
+        // that identifies rows differently silently merges or splits upstream 
records, so a
+        // mismatch has to be reported instead of looking like "nothing to 
expand".
+        validateTableKeys(
+                createTableEvent.tableId(),
+                pipelineSchema,
+                currentTargetSchema,
+                columnNameCaseSensitive,
+                plan);
+
+        for (Column pipelineColumn : pipelineSchema.getColumns()) {
+            String columnName = pipelineColumn.getName();
+            String comparisonName = normalizeColumnName(columnName, 
columnNameCaseSensitive);
+            if (ambiguousColumnNames.contains(comparisonName)) {
+                plan.incompatibilities.add(
+                        String.format(
+                                "column \"%s\" is ambiguous under the target 
system's case-sensitivity rule",
+                                columnName));
+                continue;
+            }
+
+            Column targetColumn = targetColumns.get(columnName);
+            if (targetColumn == null) {
+                if (!pipelineColumn.isPhysical() || 
keyColumns.contains(comparisonName)) {
+                    plan.incompatibilities.add(
+                            String.format(
+                                    "target table is missing the non-addable 
column \"%s\"",
+                                    columnName));
+                } else {
+                    // A brand new column is never compared against an 
existing target column, so
+                    // its type still has to be expressible by the target 
system. Without this
+                    // check the incompatibility would only surface once the 
derived DDL runs in
+                    // the apply phase, failing the job instead of being 
handled up front.
+                    Optional<DataType> normalizedNewType =
+                            normalizeType(
+                                    createTableEvent.tableId(),
+                                    columnName,
+                                    pipelineColumn.getType(),
+                                    currentTargetSchema);
+                    if (!normalizedNewType.isPresent()) {
+                        plan.incompatibilities.add(
+                                String.format(
+                                        "pipeline type %s of missing column 
\"%s\" cannot be normalized to the target type system",
+                                        pipelineColumn.getType(), columnName));
+                    } else {
+                        plan.columnsToAdd.add(
+                                
pipelineColumn.copy(pipelineColumn.getType().nullable()));
+                    }
+                }
+                continue;
+            }
+
+            Optional<DataType> normalizedPipelineTypeOptional =
+                    normalizeType(
+                            createTableEvent.tableId(),
+                            columnName,
+                            pipelineColumn.getType(),
+                            currentTargetSchema);
+            if (!normalizedPipelineTypeOptional.isPresent()) {
+                plan.incompatibilities.add(
+                        String.format(
+                                "pipeline type %s of column \"%s\" cannot be 
normalized to the target type system",
+                                pipelineColumn.getType(), columnName));
+                continue;
+            }
+            DataType normalizedPipelineType = 
normalizedPipelineTypeOptional.get().nullable();
+            DataType targetType = targetColumn.getType().nullable();
+
+            if (pipelineColumn.getType().isNullable() && 
!targetColumn.getType().isNullable()) {
+                plan.incompatibilities.add(
+                        String.format(
+                                "column \"%s\" is nullable in the pipeline but 
NOT NULL in the target table",
+                                columnName));
+                continue;
+            }
+
+            if (canContain(targetType, normalizedPipelineType)) {
+                continue;
+            }
+            if (keyColumns.contains(comparisonName)) {
+                plan.incompatibilities.add(
+                        String.format(
+                                "key column \"%s\" with target type %s cannot 
contain pipeline type %s",
+                                columnName, targetType, 
normalizedPipelineType));
+                continue;
+            }
+
+            Optional<DataType> widenedType = getSafeWidenedType(targetType, 
normalizedPipelineType);
+            if (!widenedType.isPresent()) {
+                plan.incompatibilities.add(
+                        String.format(
+                                "column \"%s\" with target type %s cannot 
safely contain pipeline type %s",
+                                columnName, targetType, 
normalizedPipelineType));
+                continue;
+            }
+
+            Optional<DataType> normalizedWidenedTypeOptional =
+                    normalizeType(
+                            createTableEvent.tableId(),
+                            columnName,
+                            widenedType.get(),
+                            currentTargetSchema);
+            if (!normalizedWidenedTypeOptional.isPresent()) {
+                plan.incompatibilities.add(
+                        String.format(
+                                "proposed widened type %s for column \"%s\" 
cannot be normalized to the target type system",
+                                widenedType.get(), columnName));
+                continue;
+            }
+            DataType normalizedWidenedType = 
normalizedWidenedTypeOptional.get().nullable();
+            if (!canContain(normalizedWidenedType, targetType)
+                    || !canContain(normalizedWidenedType, 
normalizedPipelineType)) {
+                plan.incompatibilities.add(
+                        String.format(
+                                "target system normalizes proposed widened 
type %s for column \"%s\" to %s, which is not a safe widening",
+                                widenedType.get(), columnName, 
normalizedWidenedType));
+                continue;
+            }
+
+            plan.columnsToWiden.put(
+                    targetColumn.getName(),
+                    
widenedType.get().copy(targetColumn.getType().isNullable()));
+        }
+        return plan;
+    }
+
+    private void applyPlan(CreateTableEvent createTableEvent, ExpansionPlan 
plan) {
+        if (!plan.columnsToAdd.isEmpty()) {
+            if 
(!supportsSchemaEvolutionType(SchemaChangeEventType.ADD_COLUMN)) {
+                throw new SchemaEvolveException(
+                        createTableEvent,
+                        String.format(
+                                "Target table %s is missing columns %s, but 
ADD_COLUMN is not enabled or supported by the sink.",
+                                createTableEvent.tableId(), 
getColumnNames(plan.columnsToAdd)));
+            }
+            AddColumnEvent addColumnEvent =
+                    new AddColumnEvent(
+                            createTableEvent.tableId(),
+                            plan.columnsToAdd.stream()
+                                    
.map(AddColumnEvent.ColumnWithPosition::new)
+                                    .collect(Collectors.toList()));
+            applySchemaChange(addColumnEvent, plan.columnsToAdd);
+        }
+        if (!plan.columnsToWiden.isEmpty()) {
+            if 
(!supportsSchemaEvolutionType(SchemaChangeEventType.ALTER_COLUMN_TYPE)) {
+                throw new SchemaEvolveException(
+                        createTableEvent,
+                        String.format(
+                                "Target table %s has narrow columns %s, but 
ALTER_COLUMN_TYPE is not enabled or supported by the sink.",
+                                createTableEvent.tableId(), 
plan.columnsToWiden.keySet()));
+            }
+            AlterColumnTypeEvent alterColumnTypeEvent =
+                    new AlterColumnTypeEvent(
+                            createTableEvent.tableId(),
+                            plan.columnsToWiden,
+                            plan.columnsToWiden.keySet().stream()
+                                    .collect(
+                                            Collectors.toMap(
+                                                    columnName -> columnName,
+                                                    columnName ->
+                                                            plan.targetColumns
+                                                                    
.get(columnName)
+                                                                    
.getType())));
+            applySchemaChange(alterColumnTypeEvent, plan.columnsToWiden);
+        }
+    }
+
+    /** Re-reads the target schema after applying derived DDL to detect no-op 
or failed DDL. */
+    private void verifyExpansion(
+            CreateTableEvent createTableEvent, ExpansionPlan plan, boolean 
strict) {
+        if (plan.columnsToAdd.isEmpty() && plan.columnsToWiden.isEmpty()) {
+            return;
+        }
+        Optional<Schema> updatedTargetSchema = 
queryTargetSchema(createTableEvent.tableId());
+        if (!updatedTargetSchema.isPresent()) {
+            throw new SchemaEvolveException(
+                    createTableEvent,
+                    String.format(
+                            "Failed to read back target table %s after 
expansion.",
+                            createTableEvent.tableId()));
+        }
+        ExpansionPlan remaining = analyze(createTableEvent, 
updatedTargetSchema.get());
+        if (!remaining.incompatibilities.isEmpty()
+                || !remaining.columnsToAdd.isEmpty()
+                || !remaining.columnsToWiden.isEmpty()) {
+            String message =
+                    String.format(
+                            "Target table %s still has unresolved differences 
after expansion:%s",
+                            createTableEvent.tableId(), 
describeDifferences(remaining));
+            if (strict) {
+                throw new SchemaEvolveException(createTableEvent, message);
+            }
+            LOG.warn("{}. Sink data may lose those columns.", message);
+        }
+    }
+
+    private Optional<Schema> queryTargetSchema(TableId tableId) {
+        try {
+            return expansionSupport.getExistingTableSchema(tableId);
+        } catch (Exception e) {
+            // Propagate so the job fails over and retries, instead of 
proceeding to apply the
+            // original CreateTableEvent and silently dropping columns.
+            throw new FlinkRuntimeException(
+                    "Failed to query schema of existing target table " + 
tableId, e);
+        }
+    }
+
+    private Optional<DataType> normalizeType(
+            TableId tableId,
+            String columnName,
+            DataType pipelineType,
+            Schema existingTargetSchema) {
+        try {
+            DataType normalizedType =
+                    expansionSupport.normalizeToTargetDataType(
+                            tableId, columnName, pipelineType, 
existingTargetSchema);
+            if (normalizedType == null) {
+                LOG.warn(
+                        "Target schema expansion support returned a null 
normalized type for {}.{}. Delegating this column to the sink.",
+                        tableId,
+                        columnName);
+                return Optional.empty();
+            }
+            return Optional.of(normalizedType);
+        } catch (Exception e) {
+            LOG.warn(
+                    "Failed to normalize type {} for {}.{}. Delegating this 
column to the sink.",
+                    pipelineType,
+                    tableId,
+                    columnName,
+                    e);
+            return Optional.empty();
+        }
+    }
+
+    private void applySchemaChange(SchemaChangeEvent event, Object changes) {
+        // Failures propagate to the caller: TRY_EXPAND catches and delegates 
to the sink, while
+        // CHECK/EXPAND fail the job.
+        LOG.info(
+                "Attempting to apply schema change event derived for existing 
table expansion: {} ({})",
+                event,
+                changes);
+        metadataApplier.applySchemaChange(event);
+        LOG.info(
+                "The schema change call for existing table expansion completed 
without an exception: {}",
+                event);
+    }
+
+    private boolean supportsSchemaEvolutionType(SchemaChangeEventType 
eventType) {
+        try {
+            return metadataApplier.acceptsSchemaEvolutionType(eventType)
+                    && 
metadataApplier.getSupportedSchemaEvolutionTypes().contains(eventType);
+        } catch (Exception e) {
+            LOG.warn(
+                    "Failed to determine whether {} is enabled and supported. 
Delegating schema handling to the sink.",
+                    eventType,
+                    e);
+            return false;
+        }
+    }
+
+    private static ColumnIndex indexColumns(Schema schema, boolean 
caseSensitive) {
+        Map<String, Column> columns = new HashMap<>();
+        Set<String> ambiguousColumnNames = new HashSet<>();
+        for (Column column : schema.getColumns()) {
+            String comparisonName = normalizeColumnName(column.getName(), 
caseSensitive);
+            if (ambiguousColumnNames.contains(comparisonName)) {
+                continue;
+            }
+            if (columns.putIfAbsent(comparisonName, column) != null) {
+                columns.remove(comparisonName);
+                ambiguousColumnNames.add(comparisonName);
+            }
+        }
+        return new ColumnIndex(columns, ambiguousColumnNames, caseSensitive);
+    }
+
+    private static Set<String> getKeyColumns(
+            Schema pipelineSchema, Schema targetSchema, boolean caseSensitive) 
{
+        Set<String> keyColumns = new HashSet<>();
+        pipelineSchema.primaryKeys().stream()
+                .map(columnName -> normalizeColumnName(columnName, 
caseSensitive))
+                .forEach(keyColumns::add);
+        pipelineSchema.partitionKeys().stream()
+                .map(columnName -> normalizeColumnName(columnName, 
caseSensitive))
+                .forEach(keyColumns::add);
+        targetSchema.primaryKeys().stream()
+                .map(columnName -> normalizeColumnName(columnName, 
caseSensitive))
+                .forEach(keyColumns::add);
+        targetSchema.partitionKeys().stream()
+                .map(columnName -> normalizeColumnName(columnName, 
caseSensitive))
+                .forEach(keyColumns::add);
+        return keyColumns;
+    }
+
+    /**
+     * Records primary key and partition key differences between the pipeline 
schema and the
+     * existing target table, and flags the plan as unexpandable when any is 
found.
+     *
+     * <p>Primary keys are always compared because they define how records are 
merged. Partition
+     * keys are only compared when the pipeline declares them: most sources do 
not report
+     * partitioning at all, so a two-sided comparison would make externally 
partitioned target
+     * tables look incompatible. Key names are compared as case-normalized 
sets, matching what the
+     * connectors do in their own checks, so that an ordering-only difference 
is not reported.
+     */
+    private void validateTableKeys(
+            TableId tableId,
+            Schema pipelineSchema,
+            Schema targetSchema,
+            boolean caseSensitive,
+            ExpansionPlan plan) {
+        Set<String> pipelinePrimaryKeys =
+                normalizeKeyNames(pipelineSchema.primaryKeys(), caseSensitive);
+        Set<String> targetPrimaryKeys =
+                normalizeKeyNames(targetSchema.primaryKeys(), caseSensitive);
+        if (!pipelinePrimaryKeys.equals(targetPrimaryKeys)) {

Review Comment:
   Thanks for the quick follow-up - the key comparison closes the gap I raised. 
One thing that now looks like a false positive: Paimon merges partition columns 
into the stored primary key, so a table that this pipeline created itself can 
never match the pipeline's declared PK here.
   
   `PaimonMetadataApplier#applyCreateTable` copies every partition key into 
`primaryKeys` before `catalog.createTable(...)` (asserted by 
`PaimonMetadataApplierTest`: a pipeline schema with `primaryKey("col1")` + 
`partitionKey("dt")` yields `table.primaryKeys() == [col1, dt]`), and 
`getExistingTableSchema` reads that list back verbatim. On the next run the 
comparison above sees `pipelinePrimaryKeys = {col1}` vs `targetPrimaryKeys = 
{col1, dt}` and reports a mismatch even though nothing is wrong. The same holds 
when partitioning comes from the sink's `partition.key` option, where the 
pipeline schema does not carry partition keys at all.
   
   The consequences per mode are that `CHECK`/`EXPAND` fail an otherwise valid 
job, and `TRY_EXPAND` takes the new skip path and issues no DDL, so the option 
silently does nothing for partitioned tables. I reproduced both with a target 
schema of `{col1, dt}` with PK `[col1, dt]` / partition `[dt]` against a 
pipeline schema that only adds one nullable column: `TRY_EXPAND` delegated with 
zero applied events, and `CHECK` threw with the primary-key message.
   
   Would it make sense to compare the identity key after removing partition 
columns from both sides (e.g. `targetPrimaryKeys - targetPartitionKeys` vs 
`pipelinePrimaryKeys - pipelinePartitionKeys`), or otherwise treat a target PK 
equal to `pipelinePK | pipelinePartitionKeys` as compatible? A case like this 
in `ExistingTableSchemaExpanderTest`, and ideally a partitioned table in the 
Paimon e2e, would keep the two rules from drifting apart again.



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