Added LoggingQuery for diagnostics during debug mode.

SQLite has a very nice "EXPLAIN QUERY PLAN" command (https://sqlite.org/eqp.html).
It is not really meant to be used in production code, as per the docs, but it is
super helpful at diagnosing missing indexes or other performance problems with
databases. I find it much better than, for example, the MySQL alternative.

This commit routes queries from the `ApkProvider` and `AppProvider` through a
`LoggingQuery` which (in debug builds) times queries, and if they take longer
than a certain threshold, outputs them to logcat. In addition, it will then
ask sqlite to explain its query plan for that same query, and output that to
logcat too.

app/src/main/java/org/fdroid/fdroid/data/ApkProvider.java

@@ -459,7 +459,7 @@ public class ApkProvider extends FDroidProvider {
         queryBuilder.addSelection(query);
         queryBuilder.addOrderBy(sortOrder);
 
-        Cursor cursor = db().rawQuery(queryBuilder.toString(), queryBuilder.getArgs());
+        Cursor cursor = LoggingQuery.query(db(), queryBuilder.toString(), queryBuilder.getArgs());
         cursor.setNotificationUri(getContext().getContentResolver(), uri);
         return cursor;
     }

app/src/main/java/org/fdroid/fdroid/data/AppProvider.java

@@ -785,7 +785,7 @@ public class AppProvider extends FDroidProvider {
         query.addFields(projection); // TODO: Make the order of addFields/addSelection not dependent on each other...
         query.addOrderBy(sortOrder);
 
-        Cursor cursor = db().rawQuery(query.toString(), query.getArgs());
+        Cursor cursor = LoggingQuery.query(db(), query.toString(), query.getArgs());
         cursor.setNotificationUri(getContext().getContentResolver(), uri);
         return cursor;
     }

app/src/main/java/org/fdroid/fdroid/data/LoggingQuery.java

@@ -0,0 +1,119 @@
+package org.fdroid.fdroid.data;
+
+import android.database.Cursor;
+import android.database.sqlite.SQLiteDatabase;
+
+import org.fdroid.fdroid.BuildConfig;
+import org.fdroid.fdroid.Utils;
+
+/**
+ * Helper class to log slow queries to logcat when in debug mode. When not in debug mode, it
+ * runs the queries without any logging.
+ *
+ * Here is an example of what would be output to logcat for a query that takes too long (except the
+ * query would not be formatted as nicely):
+ *
+ *   Query [155ms]:
+ *     SELECT fdroid_app.rowid as _id, ...
+ *     FROM fdroid_app
+ *       LEFT JOIN fdroid_apk ON (fdroid_apk.appId = fdroid_app.rowid)
+ *       LEFT JOIN fdroid_repo ON (fdroid_apk.repo = fdroid_repo._id)
+ *       LEFT JOIN fdroid_installedApp AS installed ON (installed.appId = fdroid_app.id)
+ *       LEFT JOIN fdroid_apk AS suggestedApk ON (fdroid_app.suggestedVercode = suggestedApk.vercode AND fdroid_app.rowid = suggestedApk.appId)
+ *     WHERE
+ *       fdroid_repo.isSwap = 0 OR fdroid_repo.isSwap IS NULL
+ *     GROUP BY fdroid_app.rowid
+ *     ORDER BY fdroid_app.name COLLATE LOCALIZED
+ *   Explain:
+ *     SCAN TABLE fdroid_app
+ *     SEARCH TABLE fdroid_apk USING COVERING INDEX sqlite_autoindex_fdroid_apk_1 (appId=?)
+ *     SEARCH TABLE fdroid_repo USING INTEGER PRIMARY KEY (rowid=?)
+ *     SEARCH TABLE fdroid_installedApp AS installed USING INDEX sqlite_autoindex_fdroid_installedApp_1 (appId=?)
+ *     SEARCH TABLE fdroid_apk AS suggestedApk USING INDEX sqlite_autoindex_fdroid_apk_1 (appId=? AND vercode=?)
+ *     USE TEMP B-TREE FOR ORDER BY
+ */
+public class LoggingQuery {
+
+    private static final long SLOW_QUERY_DURATION = 100;
+    private static final String TAG = "Slow Query";
+
+    private final SQLiteDatabase db;
+    private final String query;
+    private final String[] queryArgs;
+
+    public LoggingQuery(SQLiteDatabase db, String query, String[] queryArgs) {
+        this.db = db;
+        this.query = query;
+        this.queryArgs = queryArgs;
+    }
+
+    /**
+     * When running a debug build, this will log details (including query plans) for any query which
+     * takes longer than {@link LoggingQuery#SLOW_QUERY_DURATION}.
+     */
+    public Cursor rawQuery() {
+        if (BuildConfig.DEBUG) {
+            long startTime = System.currentTimeMillis();
+            Cursor cursor = db.rawQuery(query, queryArgs);
+            long queryDuration = System.currentTimeMillis() - startTime;
+
+            if (queryDuration >= SLOW_QUERY_DURATION) {
+                logSlowQuery(queryDuration);
+            }
+            return cursor;
+        } else {
+            return db.rawQuery(query, queryArgs);
+        }
+    }
+
+    /**
+     * Log the query and its duration to the console. In addition, execute an "EXPLAIN QUERY PLAN"
+     * for the query in question so that the query can be diagnosed (https://sqlite.org/eqp.html)
+     */
+    private void logSlowQuery(long queryDuration) {
+        StringBuilder sb = new StringBuilder();
+        sb.append("Query [")
+                .append(queryDuration)
+                .append("ms]: ")
+                .append(query);
+
+        try {
+            StringBuilder sbExplain = new StringBuilder("\nExplain:\n");
+            for (String plan : getExplainQueryPlan()) {
+                sbExplain.append("  ").append(plan).append("\n");
+            }
+            sb.append(sbExplain);
+        } catch (Exception e) {
+            // Ignore exception, we caught this because the SQLite docs say explain query plan can
+            // change between versions. We do our best in getExplainQueryPlan() to mitigate this,
+            // but it may still break with newer releases. In that case, at least we will still be
+            // logging the slow query to logcat which is helpful.
+        }
+
+        Utils.debugLog(TAG, sb.toString());
+    }
+
+    private String[] getExplainQueryPlan() {
+        Cursor cursor = db.rawQuery("EXPLAIN QUERY PLAN " + query, queryArgs);
+        String[] plan = new String[cursor.getCount()];
+        cursor.moveToFirst();
+        while (!cursor.isAfterLast()) {
+            // The docs at https://sqlite.org/eqp.html talk about how the output format of
+            // EXPLAIN QUERY PLAN can change between SQLite versions. This has been observed
+            // between the sqlite versions on Android 2.3.3 and Android 5.0. However, it seems
+            // that the last column is always the one with the interesting details that we wish
+            // to log. If this fails for some reason, then hey, it is only for debug builds, right?
+            if (cursor.getColumnCount() > 0) {
+                int index = cursor.getColumnCount() - 1;
+                plan[cursor.getPosition()] = cursor.getString(index);
+            }
+            cursor.moveToNext();
+        }
+        cursor.close();
+        return plan;
+    }
+
+    public static Cursor query(SQLiteDatabase db, String query, String[] queryBuilderArgs) {
+        return new LoggingQuery(db, query, queryBuilderArgs).rawQuery();
+    }
+}