System.QueryException in Apex SOQL query row limit

Answer

The System.QueryException you see is thrown when the Apex transaction exceeds the governor limit for the number of SOQL queries, not when the row limit is exceeded.

Confirmed facts

• The synchronous SOQL query limit is a fixed 100 queries per transaction (200 in certain asynchronous contexts). It does not scale with heap size; heap (≈6 MB sync, 12 MB async) is an independent limit.
• The 50 000‑row limit is cumulative across all SOQL queries in the transaction. Rows returned by relationship subqueries, aggregate queries, and normal queries all count toward this total, so deeper relationship nesting consumes the row budget faster but does not change the threshold itself.
• Exceeding the query limit throws System.QueryException; exceeding the row limit throws System.LimitException (uncatchable). Both are governor‑limit exceptions that cannot be handled in Apex.
• Salesforce enforces the same documented limits in full‑copy sandboxes and production orgs for the same edition. Apparent differences usually stem from data volume, not from a different enforcement algorithm.

Likely explanation for intermittent failures

In a test class the data set is small, so the cumulative row count stays well below 50 000 and the query count stays below 100. In production (or a full‑copy sandbox loaded with realistic data) the same logic may iterate over many records, causing the query count to creep past 100 or the row total to surpass 50 000, triggering the exception.

Steps to resolve / prevent

1. Move SOQL out of loops: collect IDs in a Set/List, query once, then use a Map for look‑ups.
2. If you need child‑parent data, use a single query with relationship subqueries (parent‑to‑child) or perform two separate queries and combine in Apex.
3. Use aggregate queries (COUNT, GROUP BY) when you only need totals; they still count rows but are often cheaper.
4. Check headroom during development with Limits methods:

   Integer queriesUsed   = Limits.getQueries();
   Integer queryLimit    = Limits.getLimitQueries();
   Integer rowsUsed      = Limits.getQueryRows();
   Integer rowLimit      = Limits.getLimitQueryRows();
   System.debug('Queries: '+queriesUsed+'/'+queryLimit);
   System.debug('Rows:   '+rowsUsed+'/'+rowLimit);
   

   Add these debug statements around the suspect code path and review the log in a full‑copy sandbox that contains production‑scale data.

Missing diagnostic detail

To confirm whether the exception is due to the query‑count limit or the row‑count limit, please share the debug log excerpt that shows the Number of SOQL queries and Number of query rows values at the point of failure. Knowing which counter is exceeded will tell you whether to focus on reducing query calls or on lowering the cumulative row volume.