[FLINK-39409][connect/postgres] Support tables without primary key for incremental snapshot

[JNSimba]

Summary

This PR adds support for PostgreSQL tables without primary keys in incremental snapshot mode,
following the same approach as MySQL (PR #2150).

Changes

JDBC Base Layer (flink-cdc-base):

• SplitKeyUtils.getSplitKey(): Handle null record key by extracting chunk key from value's
  after/before struct instead of record key
• IncrementalSourceScanFetcher.pollWithBuffer(): Use after struct as buffer key when
  record key is null (no-PK table)
• JdbcSourceFetchTaskContext.rewriteOutputBuffer(): Support no-PK merge logic — for tables
  without primary key, use before/after struct as buffer key for CREATE/UPDATE/DELETE operations

PostgreSQL Connector (flink-connector-postgres-cdc):

• PostgresDialect: Validate that tables without primary key must have REPLICA IDENTITY FULL
  set, querying pg_class.relreplident and failing fast with a clear error message if not

Tests:

• Added products_no_pk table DDL (without PK, with REPLICA IDENTITY FULL)
• testNoPKTableWithChunkKey: End-to-end test for no-PK table with scan.incremental.snapshot.chunk.key-column configured
• testNoPKTableWithoutChunkKey: Verify ValidationException is thrown when chunk key column is not specified

Design Decisions

• No interface change: Kept Map<Struct, SourceRecord> in FetchTask.Context.rewriteOutputBuffer()
  unchanged to avoid breaking MongoDB and other connectors. For no-PK tables, the full row struct
  (all columns via REPLICA IDENTITY FULL) serves as the buffer key.
• At-least-once for chunk key updates: When an UPDATE changes the chunk key column value and
  crosses split boundaries, only at-least-once semantics can be guaranteed (consistent with MySQL behavior).
• REPLICA IDENTITY FULL required: PostgreSQL requires this setting on no-PK tables to provide
  full before image in WAL for UPDATE/DELETE events. The connector validates this at startup.

[Copilot]

Pull request overview

Adds support for PostgreSQL tables without primary keys when running in incremental snapshot mode, aligning behavior with the existing MySQL approach.

Changes:

• Extend incremental snapshot split-key extraction and output buffering to handle Debezium records with null keys (no-PK tables).
• Add PostgreSQL-side validation to require REPLICA IDENTITY FULL for no-PK tables.
• Add end-to-end Postgres IT coverage and new DDL for a no-PK test table.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
flink-cdc-connect/flink-cdc-source-connectors/flink-connector-postgres-cdc/src/test/resources/ddl/inventory_no_pk.sql	Adds a no-PK test table and seed data (with REPLICA IDENTITY FULL).
flink-cdc-connect/flink-cdc-source-connectors/flink-connector-postgres-cdc/src/test/java/org/apache/flink/cdc/connectors/postgres/table/PostgreSQLConnectorITCase.java	Adds IT cases for no-PK tables with/without chunk key.
flink-cdc-connect/flink-cdc-source-connectors/flink-connector-postgres-cdc/src/main/java/org/apache/flink/cdc/connectors/postgres/source/PostgresDialect.java	Validates REPLICA IDENTITY FULL for no-PK tables during schema discovery.
flink-cdc-connect/flink-cdc-source-connectors/flink-cdc-base/src/main/java/org/apache/flink/cdc/connectors/base/utils/SplitKeyUtils.java	Extracts chunk key from value (before/after) when record key is null.
flink-cdc-connect/flink-cdc-source-connectors/flink-cdc-base/src/main/java/org/apache/flink/cdc/connectors/base/source/reader/external/JdbcSourceFetchTaskContext.java	Supports buffer rewrite/merge logic for no-PK by using before/after structs as buffer keys.
flink-cdc-connect/flink-cdc-source-connectors/flink-cdc-base/src/main/java/org/apache/flink/cdc/connectors/base/source/reader/external/IncrementalSourceScanFetcher.java	Uses value after struct as initial snapshot buffer key when record key is null.

Comments suppressed due to low confidence (1)

flink-cdc-connect/flink-cdc-source-connectors/flink-cdc-base/src/main/java/org/apache/flink/cdc/connectors/base/source/reader/external/JdbcSourceFetchTaskContext.java:142

• Typo/grammar in exception message: "the the record" should be corrected to "the record" (and/or rephrase for clarity).

                    throw new IllegalStateException(
                            String.format(
                                    "Data change record shouldn't use READ operation, the the record is %s.",
                                    changeRecord));

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