[DocDB] YSQL parallel query execution

[ttyusupov]

Jira Link: DB-7047

Description

PostgreSQL can devise query plans that can leverage multiple CPUs in order to answer queries faster. This feature is known as parallel query. For queries, when a large number of rows are scanned this helps improve the query performance by utilizing parallel workers. In YugabyteDB, we plan to support Parallel query to improve the performance execution of the queries.

1. [:clock4: ] ([DocDB] Add docdb support for dynamic/interleaved parallelism every N blocks #17986)
   • [DocDB] Implement GetTableKeyRanges API limited to range-sharded tables support and forward scan #19341
   • [DocDB] Implement backward scan support for GetTableKeyRanges API #19342
   • [DocDB] Implement support for GetTableKeyRanges API for hash sharded tables #19343
2. [ 🕓] ([YSQL] YSQL layer - parallel query execution #17987)
   • [YSQL] Add Support for PostgreSQL parallelism for YbSeqScan plan node  #18095
   • [YSQL] Make YbSeqScan plan node as default for sequential scan  #18082
     • Support YbSeqScan aggregate pushdown([YSQL] Aggregates are not pushed down to docdb layer in case of index scan #15857)
   • [YSQL] Support for PostgreSQL parallelism for IndexScan/IndexOnlyScans  #18094
   • [YSQL] Support for PostgreSQL parallelism for Parallel Append plan node #18092
   • [YSQL] Support for PostgreSQL parallelism for Aggregation plan node #18093

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