Count Mismatch Challenges During PostgreSQL Database Migration: Causes and Solutions

Question

Version - PostgreSQL 10.20, compiled by Visual C++ build 1800, 64-bit

Platform - Windows

Regarding PostgreSQL, during the migration from PG 10.20 to 14.7, we will perform a sanity check on the migrated tables using source and destination row counts. Occasionally, we encounter count mismatches where the source database has fewer rows, resulting in a higher count, or more rows, resulting in a lower count.

Note: There are no migration issues, as all data rows have been successfully migrated without data loss.

ProdDB=> select count(*) from PdtDetailsTable;
  count
---------
 1297324
(1 row)


ProdDB=> reindex table PdtDetailsTable;
REINDEX
ProdDB=> select count(*) from PdtDetailsTable;
  count
---------
 1297322
(1 row)


ProdDB=>

Upon reindexing, the count(*) query returns the correct results in the source DB.

In the pg_log, we couldn't find any traces related to this behavior. Is there a reason for such behavior, and how can this be addressed to prevent future issues?