Question

我正在使用Objective C在iOS上使用键盘扩展，我正在使用SQLite。我需要了解一些关于SQLite的概念，我没有通过谷歌搜索获得。让我将问题分成几部分。

PART：1

我在SQLite中遇到了一个名为PRAGMA cache_size = pages;

的PRAGMA

此处的默认大小为2000页。 根据我的理解，与默认相比，

cache_size＆gt; 2000意味着更多的内存使用，更快的速度（默认）。
cache_size＆lt; 2000意味着更少的内存使用，更低的速度（默认）。

我在这里纠正吗？

PART：2

我正在尝试以下列方式更改cache_size，

if (sqlite3_exec(sqlite3Database, [@"PRAGMA CACHE_SIZE=50;" UTF8String], NULL, NULL, NULL) == SQLITE_OK) {
        NSLog(@"Successfully changed cache size");
    }
    else
        NSLog(@"Error: failed to set cache size with message %s.", sqlite3_errmsg(sqlite3Database));

打开数据库后我正在使用它。以下代码显示了它，

-(void)runQuery:(const char *)query isQueryExecutable:(BOOL)queryExecutable{
// Create a sqlite object.
sqlite3 *sqlite3Database;

// Set the database file path.
NSString *databasePath = [self getDbFilePath];

// Initialize the results array.
if (self.arrResults != nil) {
    [self.arrResults removeAllObjects];
    self.arrResults = nil;
}
self.arrResults = [[NSMutableArray alloc] init];


// Open the database.
BOOL openDatabaseResult = sqlite3_open([databasePath UTF8String], &sqlite3Database);
if(openDatabaseResult == SQLITE_OK) {

    if (sqlite3_exec(sqlite3Database, [@"PRAGMA CACHE_SIZE=50;" UTF8String], NULL, NULL, NULL) == SQLITE_OK) {
        NSLog(@"Successfully changed cache size");
    }
    else
        NSLog(@"Error: failed to set cache size with message %s.", sqlite3_errmsg(sqlite3Database));
    // Declare a sqlite3_stmt object in which will be stored the query after having been compiled into a SQLite statement.
    sqlite3_stmt *compiledStatement;

    // Load all data from database to memory.
    BOOL prepareStatementResult = sqlite3_prepare_v2(sqlite3Database, query, -1, &compiledStatement, NULL);
    if(prepareStatementResult == SQLITE_OK) {
        // Check if the query is non-executable.
        if (!queryExecutable){
            // In this case data must be loaded from the database.

            // Declare an array to keep the data for each fetched row.
            NSMutableArray *arrDataRow;

            // Loop through the results and add them to the results array row by row.
            while(sqlite3_step(compiledStatement) == SQLITE_ROW) {
                // Initialize the mutable array that will contain the data of a fetched row.
                arrDataRow = [[NSMutableArray alloc] init];

                // Get the total number of columns.
                int totalColumns = sqlite3_column_count(compiledStatement);

                // Go through all columns and fetch each column data.
                for (int i=0; i<totalColumns; i++){
                    // Convert the column data to text (characters).
                    char *dbDataAsChars = (char *)sqlite3_column_text(compiledStatement, i);

                    // If there are contents in the currenct column (field) then add them to the current row array.
                    if (dbDataAsChars != NULL) {
                        // Convert the characters to string.
                        [arrDataRow addObject:[NSString  stringWithUTF8String:dbDataAsChars]];
                    }
                }

                // Store each fetched data row in the results array, but first check if there is actually data.
                if (arrDataRow.count > 0) {
                    [self.arrResults addObject:arrDataRow];
                }
            }
        }
        else {
            // This is the case of an executable query (insert, update, ...).

            // Execute the query.
            int executeQueryResults = sqlite3_step(compiledStatement);
            if (executeQueryResults == SQLITE_DONE) {
                // Keep the affected rows.
                self.affectedRows = sqlite3_changes(sqlite3Database);

                // Keep the last inserted row ID.
                self.lastInsertedRowID = sqlite3_last_insert_rowid(sqlite3Database);
            }
            else {
                // If could not execute the query show the error message on the debugger.
                NSLog(@"DB Error: %s", sqlite3_errmsg(sqlite3Database));
            }
        }
    }
    else {
        // In the database cannot be opened then show the error message on the debugger.
        NSLog(@"db error: %s", sqlite3_errmsg(sqlite3Database));
    }

    // Release the compiled statement from memory.
    sqlite3_finalize(compiledStatement);

}

// Close the database.
sqlite3_close(sqlite3Database);

}

但是，当我调用方法sqlite3_exec(sqlite3Database, [@"PRAGMA CACHE_SIZE=50;" UTF8String], NULL, NULL, NULL)时，无论我做什么，它总是会SQLITE_OK。

例如，如果我sqlite3_exec(sqlite3Database, [@"abcd bla bla" UTF8String], NULL, NULL, NULL)，则会返回SQLITE_OK !!

为什么会这样？

PART：3

我想提高查询的执行速度，但同时又不想使用IMDB，因为数据库的大小很大。

PRAGMA page_size = bytes;在这种情况下是否有任何相关性？如果是，那么如何在Objective C中做到这一点？

感谢任何帮助。谢谢和问候。

SQLite PRAGMA cache_size iOS

0 个答案: