branches/zip: page_zip_empty_size(): New function to compute the

guaranteed free space available for inserting one record.

btr_page_get_sure_split_rec(), btr_cur_pessimistic_insert():
Use page_zip_empty_size().

btr_page_split_and_insert(): Relax a debug assertion that there should
be at least two user records on the page.  On compressed pages, we may
be able to write only one record.

btr/btr0btr.c

@@ -1332,14 +1332,12 @@ btr_page_get_sure_split_rec(
 
 	page_zip = buf_block_get_page_zip(buf_block_align(page));
 	if (UNIV_LIKELY_NULL(page_zip)) {
-		/* Estimate the free space of an empty compressed page.
-		The space needed for compressing the index information
-		is estimated. */
-		ulint	free_space_zip = page_zip->size
-			- PAGE_DATA - cursor->index->n_fields / 2;
-
-		if (UNIV_LIKELY(free_space > free_space_zip)) {
-			free_space = free_space_zip;
+		/* Estimate the free space of an empty compressed page. */
+		ulint	free_space_zip = page_zip_empty_size(
+			cursor->index->n_fields, page_zip->size);
+
+		if (UNIV_LIKELY(free_space > (ulint) free_space_zip)) {
+			free_space = (ulint) free_space_zip;
 			ut_a(insert_size <= free_space);
 		}
 	}
@@ -1746,7 +1744,7 @@ btr_page_split_and_insert(
 
 	ut_ad(mtr_memo_contains(mtr, buf_block_align(page),
 				MTR_MEMO_PAGE_X_FIX));
-	ut_ad(page_get_n_recs(page) >= 2);
+	ut_ad(page_get_n_recs(page) >= 1);
 
 	page_no = buf_frame_get_page_no(page);
 

btr/btr0cur.c

@@ -1341,11 +1341,9 @@ btr_cur_pessimistic_insert(
 	}
 
 	if (UNIV_UNLIKELY(zip_size)) {
-		/* Estimate the free space of an empty compressed page.
-		The space needed for compressing the index information
-		is estimated. */
-		ulint	free_space_zip = zip_size
-			- PAGE_DATA - cursor->index->n_fields / 2;
+		/* Estimate the free space of an empty compressed page. */
+		ulint	free_space_zip = page_zip_empty_size(
+			cursor->index->n_fields, zip_size);
 
 		if (UNIV_UNLIKELY(rec_get_converted_size(index, entry)
 				  > free_space_zip)) {

include/page0zip.h

@@ -32,6 +32,17 @@ page_zip_rec_needs_ext(
 	ulint	zip_size)	/* in: compressed page size in bytes, or 0 */
 	__attribute__((const));
 
+/**************************************************************************
+Determine the guaranteed free space on an empty page. */
+
+ulint
+page_zip_empty_size(
+/*================*/
+				/* out: minimum payload size on the page */
+	ulint	n_fields,	/* in: number of columns in the index */
+	ulint	zip_size)	/* in: compressed page size in bytes */
+	__attribute__((const));
+
 /**************************************************************************
 Initialize a compressed page descriptor. */
 UNIV_INLINE

page/page0zip.c

@@ -52,6 +52,30 @@ static const byte supremum_extra_data[] = {
 static const byte zero[BTR_EXTERN_FIELD_REF_SIZE] = { 0, };
 #endif
 
+/**************************************************************************
+Determine the guaranteed free space on an empty page. */
+
+ulint
+page_zip_empty_size(
+/*================*/
+				/* out: minimum payload size on the page */
+	ulint	n_fields,	/* in: number of columns in the index */
+	ulint	zip_size)	/* in: compressed page size in bytes */
+{
+	lint	size = zip_size
+		/* subtract the page header and the longest
+		uncompressed data needed for one record */
+		- (PAGE_DATA
+		   + PAGE_ZIP_DIR_SLOT_SIZE
+		   + DATA_TRX_ID_LEN + DATA_ROLL_PTR_LEN
+		   + 1/* encoded heap_no==2 in page_zip_write_rec() */
+		   + 1/* end of modification log */
+		   - REC_N_NEW_EXTRA_BYTES/* omitted bytes */)
+		/* subtract the space for page_zip_fields_encode() */
+		- compressBound(2 * (n_fields + 1));
+	return(size > 0 ? (ulint) size : 0);
+}
+
 /*****************************************************************
 Gets the size of the compressed page trailer (the dense page directory),
 including deleted records (the free list). */