Fix cache_empty_result=0 not caching non-empty resultsets (issue #5248)

[renecannao]

Summary

Fixes issue #5248: Setting cache_empty_result to "0" on individual mysql_query_rules doesn't work.

Problem: When cache_empty_result is set to "0" on a query rule, nothing gets cached at all (even non-empty resultsets). According to the issue reporter, they want to disable caching for empty resultsets but still cache non-empty resultsets.

Root Cause: The caching logic in MySQL_Session.cpp and PgSQL_Session.cpp only handled two cases:

1. cache_empty_result == 1: Always cache
2. cache_empty_result == -1: Use global setting OR cache if rows > 0
   Missing case: cache_empty_result == 0: Cache only if rows > 0

Solution: Added the missing condition (qpo->cache_empty_result == 0 && MyRS->num_rows) (MySQL) / (qpo->cache_empty_result == 0 && num_rows) (PgSQL) to allow caching when cache_empty_result=0 AND the resultset has rows.

Changes:

1. MySQL_Session.cpp (lines 7495-7502): Added condition for cache_empty_result == 0
2. PgSQL_Session.cpp (lines 4815-4821): Added condition for cache_empty_result == 0
3. query_processor.h: Added extensive Doxygen documentation for the cache_empty_result field explaining its three possible values
4. Query_Processor.cpp: Added inline comments clarifying the three possible values

Semantics of cache_empty_result:

• -1: Use global setting (query_cache_stores_empty_result)
• 0: Do NOT cache empty resultsets, but cache non-empty resultsets
• 1: Always cache resultsets (both empty and non-empty)

Testing Considerations: The fix maintains backward compatibility:

• cache_empty_result=1: Still caches everything
• cache_empty_result=-1: Still uses global setting OR caches non-empty results
• cache_empty_result=0: Now correctly caches non-empty results (previously cached nothing)

References:

• Issue: Setting cache_empty_result to "0" on individual mysql_query_rules doesn't work #5248
• Affected files: lib/MySQL_Session.cpp:7497, lib/PgSQL_Session.cpp:4817

[gemini-code-assist]

Summary of Changes

Hello @renecannao, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a long-standing bug in the query caching mechanism where the cache_empty_result setting for query rules was not behaving as intended. Previously, configuring a rule to not cache empty resultsets (cache_empty_result=0) inadvertently disabled caching entirely for that rule. The changes rectify this by modifying the core caching logic in both MySQL and PostgreSQL session handlers to correctly differentiate between empty and non-empty resultsets when this setting is applied, ensuring that only empty results are excluded from the cache. Additionally, comprehensive documentation has been integrated to clarify the parameter's behavior for future development and maintenance.

Highlights

• Fix for cache_empty_result=0 behavior: Resolved an issue where setting cache_empty_result to 0 on individual query rules incorrectly prevented all resultsets (including non-empty ones) from being cached. The intended behavior for cache_empty_result=0 is to only prevent empty resultsets from being cached, while still caching non-empty ones.
• Updated Caching Logic: The caching logic in MySQL_Session.cpp and PgSQL_Session.cpp has been updated to correctly handle the cache_empty_result=0 setting, ensuring that non-empty resultsets are now cached as expected.
• Enhanced Documentation: Extensive Doxygen documentation has been added to query_processor.h and inline comments to Query_Processor.cpp, MySQL_Session.cpp, and PgSQL_Session.cpp to clearly define the three possible values and semantics of the cache_empty_result field (-1, 0, 1).

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[gemini-code-assist]

Code Review

This pull request addresses issue #5248 by fixing the behavior of the cache_empty_result setting in query rules. Previously, setting cache_empty_result to 0 would prevent all caching, even for non-empty result sets. The changes update the caching logic in MySQL_Session.cpp and PgSQL_Session.cpp to ensure that when cache_empty_result is 0, only non-empty result sets are cached, while empty ones are not. Additionally, detailed Doxygen-style comments have been added to include/query_processor.h and lib/Query_Processor.cpp to clarify the meaning of the cache_empty_result values (-1 for global setting, 0 for caching non-empty only, and 1 for always caching).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud