feat: implement PostgreSQL data extraction and transformation process

cmd/go_migrate/extractor.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"database/sql"
 
+	"github.com/jackc/pgx/v5/pgxpool"
 	_ "github.com/microsoft/go-mssqldb"
 	log "github.com/sirupsen/logrus"
 )
@@ -50,3 +51,38 @@ func extractFromMssql(ctx context.Context, job MigrationJob, columns []ColumnTyp
 
 	return nil
 }
+
+func extractFromPostgres(ctx context.Context, job MigrationJob, columns []ColumnType, chunkSize int, db *pgxpool.Pool, out chan<- []UnknownRowValues) error {
+	query := buildExtractQueryPostgres(job, columns)
+	log.Debug("Query used to extract data from postgres: ", query)
+
+	rows, err := db.Query(ctx, query)
+	if err != nil {
+		return err
+	}
+	defer rows.Close()
+
+	rowsChunk := make([]UnknownRowValues, 0, chunkSize)
+
+	for rows.Next() {
+		values, err := rows.Values()
+		if err != nil {
+			return err
+		}
+
+		rowsChunk = append(rowsChunk, values)
+
+		if len(rowsChunk) >= chunkSize {
+			out <- rowsChunk
+			rowsChunk = make([]UnknownRowValues, 0, chunkSize)
+			log.Infof("Chunk send... %+v", job)
+		}
+	}
+
+	if len(rowsChunk) > 0 {
+		out <- rowsChunk
+		log.Infof("Chunk send... %+v", job)
+	}
+
+	return nil
+}